These are not the monsters exactly as I ran them; rather, they are updated versions based on my own experience with running them. They in general run on the more damaging side (as my players noticed) but also ideally of lower health and defenses, to get as close to L4D's greater swinginess (to use an RPG term) while still feeling like 4E. House rules that already do this (e.g. decreasing monster hit points and increasing their damage) will amplify this effect.

Zombie Horde Level 3 Brute
Huge Natural Humanoid (undead, swarm) XP 150
Initiative +1 Senses Perception +1; darkvision
Grabbing Mass aura 1; at the start its turn the zombie horde makes a melee basic attack against each enemy in the aura as a free action.
HP 55; Bloodied 27
AC 15; Fortitude 18, Reflex 15, Will 15
Immune disease, poison; Resist half damage from melee and ranged attacks, 10 necrotic; Vulnerable 5 close and area attacks, 5 radiant, 5 fire
Saving Throws +5 to effects imposed by ranged or melee attacks
Speed 6 squares, climb 4
M Slam (Standard; at-will)
Melee 1; +6 vs. AC; 2d6 + 3 damage.
m Grab (Standard; at-will)
Melee 1; +4 vs. Reflex; 1d10 + 3 damage, and the target is grabbed (until escape). Attempts to escape the zombie horde's grab take a -5 penalty.
m Engulf (against a grabbed target, standard; at-will)
Melee 1; +4 vs Fortitude; 2d6 + 3 damage, and the target is engulfed by the zombie horde (until escape). While engulfed the target is grabbed, restrained, takes ongoing 10 damage (ongoing 5 while the zombie horde is bloodied), and slides into an adjacent square occupied by the zombie horde. An attempt to escape being engulfed takes a -5 penalty (not cumulative with the zombie horde's Grab penalty). Any ranged or melee attack against the target instead targets the zombie horde. While engulfing a target, the zombie horde can take no action other than to sustain the engulfment, although its aura still functions.
Horde Stability
The zombie horde can reduce any push, pull, or slide effect against it by 1 square.
Thinning Out (when first bloodied)
The zombie horde's size changes to Large, and all its attacks take a -2 penalty to damage.
Stragglers (when reduced to 0 hit points)
Three zombie rotters appear in any unoccupied space formerly occupied by the zombie horde.
Alignment Unaligned Languages --
Str 16 (+4) Dex 11 (+1) Wis 11 (+1)
Con 15 (+3) Int 3 (-3) Cha 3 (-3)

The zombie horde owes much of its design to a similar homebrew monster on the EN World forums by a user whose name I unfortunately do not recall. It begins as a fairly normal swarm creature (though unusual in that it is a swarm of Medium creatures) with an unusual and powerful Engulf ability that unfortunately I never got to try out in actual combat. The Engulf ability is meant to simulate getting surrounded by the horde in L4D and is quite dangerous, although as a brute the attack is unlikely to hit.

One of the additions to most of these monsters from the versions I ran is a vulnerability to fire. When the tank attacked my players their first thought was to set it on fire; I had not originally included any rules for it, but I decided on the fly to give the tank vulnerability to fire (although not actually catch on fire from such damage, to my players' disappointment), which I then transferred over to the rest of the zombies as I went over them afterwards. Purely from a 4E stand point, the radiant vulnerability is completely servicable and adding fire gives them an additional weakness, but fire is such an iconic part of L4D that I didn't want to give it up.

Zombie Hunter Level 3 Lurker
Medium Natural Humanoid (undead) XP 150
Initiative +8 Senses Perception +8; darkvision
HP 37; Bloodied 18
AC 17; Fortitude 16, Reflex 18, Will 16
Immune disease, poison; Resist 10 necrotic; Vulnerable 5 radiant, 5 fire
Speed 8 squares, climb 6 (spider climb)
M Claw (Standard; at-will)
Melee 1; +8 vs. AC; 2d6 + 3 damage.
m Pounce (only usable as part of a charge; standard; at-will)
Melee 1; +6 vs. Reflex; 1d6 + 3 damage, ongoing 5 damage, and the target is grabbed and dazed (escape ends all) and knocked prone. Attempts to escape the zombie hunter's grab take a -5 penalty. The zombie hunter enters the target's space, is dazed, and any melee or ranged attack against the target instead targets the zombie hunter. If the zombie hunter leaves the target's space for any reason, the grab ends.
Leaper
As a move action or as part of a charge, the zombie hunter can jump up to its speed without making an Athletics check and gains a +4 bonus to defenses against opportunity attacks provoked by jumping.
Flaming Hunter
If the zombie hunter takes fire damage, its attacks on its next turn gain a +2 bonus to damage (including ongoing damage).
Alignment Unaligned Languages --
Skills Stealth +9, Acrobatics +9
Str 14 (+3) Dex 17 (+4) Wis 14 (+3)
Con 13 (+2) Int 4 (-2) Cha 3 (-3)

Hunters were especially interesting to design due to their Pounce attack. I borrowed from the horde's already established Engulf ability for an attack that did a lot of damage if sustained (though, like the actual L4D hunter, the initial damage is less than a simple claw swipe) but also made the hunter more vulnerable. (In one encounter, I had a horde attack a hunter pounced on a character due to its aura; I figured it fit the mindlessness of the Horde.)

Flaming Hunter is one of my favorite features of any monster I've ever designed. Part of it is that I like the idea of mixing advantages and dangers--hit the hunter with fire to do more damage to it, but also make it do more damage. And part of it is simply for the fact that flaming hunters are probably my favorite part of L4D, to the point that I giddily scream "Flaming hunter!" and intentionally set myself on fire when playing as one. (I suppose a few people might not actually know: hunters in L4D do significantly more damage when on fire. They also don't take ongoing damage from fire while pouncing someone, which makes setting yourself on fire extremely effective in most situations.)

Zombie Smoker Level 3 Controller
Medium Natural Humanoid (undead) XP 150
Initiative +4 Senses Perception +3; darkvision
HP 43; Bloodied 21
AC 17; Fortitude 16, Reflex 17, Will 16
Immune disease, poison; Resist 10 necrotic; Vulnerable 5 radiant, 5 fire
Speed 6 squares, climb 5
M Claw (Standard; at-will)
Melee 1; +8 vs AC; 1d6 + 3 damage.
r Tongue Grab (Standard; recharge 56)
Ranged 10; +7 vs Reflex; 2d6 + 3 damage, ongoing 5 (until escape, and the target is knocked prone, pulled 3, and grabbed (until escape). Attempts to escape the zombie smoker's grab take a -5 penalty.
Sustain Standard: the target is pulled 3 squares. The first time the zombie smoker sustains this power, the target is dazed (until escape). The second time the zombie smoker sustains this power, the target becomes unconscious (until the target takes damage from an attack). If the zombie smoker fails to sustain this power, the target automatically escapes at the end of the zombie smoker's turn. The zombie smoker can only recharge this power when it does not have a target grabbed.
c Smoking Cloud (when the zombie smoker is reduced to 0 hit points) ♦ poison
Close burst 1; +7 vs Fortitude; 1d6 + 3 poison damage, and the target is weakened and takes a -2 penalty to attack rolls until the end of its next turn.
Alignment Unaligned Languages --
Skills Stealth +9
Str 14 (+3) Dex 17 (+4) Wis 14 (+3)
Con 11 (+1) Int 5 (-2) Cha 3 (-3)

The smoker is my favorite of L4D's zombies, and it proved the most annoying of any of the zombies in the 4E version due to its ability to attack at range and quickly disappear. The smoker is labeled here as a Controller, but that's mostly for diversity: all of the specials are clearly Lurkers, with the boomer having an obvious Leader ability as well, but smokers clearly have a minor in controlling as well--their job, after all, is not so much damage as disruption of the survivors' formation.

While the smoker proved the most annoying of foes for my players, it never managed to sustain its grab long enough to reach the more powerful effects (it kept strangling the ranger, who had a very high Acrobatics modifier), so I'm not sure how those work out. Obviously it's very powerful for a level 3 monster, but I also ruled that players could use their own move action to make an escape check for the grabbed player (essentially L4D's melee-to-free), which I don't think is RAW an option but seems reasonable.

Zombie Boomer Level 2 Lurker (Leader)
Medium Natural Humanoid (undead) XP 125
Initiative +5 Senses Perception +1; darkvision
Boomer Stench aura 1; enemies in the aura take a -2 penalty to attack rolls.
HP 26; Bloodied 13
AC 15; Fortitude 13, Reflex 13, Will 14
Immune disease, poison; Resist 10 necrotic; Vulnerable 5 radiant, 5 force
Speed 5 squares
M Claw (Standard; at-will)
Melee 1; +7 vs. AC; 1d6 + 3 damage.
c Vomit (Standard; recharge 6) ♦ Acid
Close blast 3; +5 vs Reflex; 1d10 + 3 acid damage, and the target is blinded (save ends). While a target is blinded, zombies gain a +4 bonus to damage rolls against the target.
c Boomer Burst (when the zombie boomer is reduced to 0 hit points) ♦ Acid
Close burst 2; the zombie boomer makes a Vomit attack against each non-zombie creature in range.
Alignment Unaligned Languages --
Str 13 (+2) Dex 11 (+1) Wis 11 (+1)
Con 8 (+0) Int 3 (-3) Cha 3 (-3)

Boomers are a finicky beast. Due to the way hit points work they need to be of lower level (or have a general vulnerability, I suppose, or make them minions, which would be hilarious) than the other zombies to make them seem easier to kill. I also didn't get a chance to see how their signature vomit interacted with zombies in play; the boomer's only appearance in combat only hit one player and the only other zombie around, a horde, was blocked by the rest of the characters.

Zombie Tank Level 3 Elite Brute
Large Natural Humanoid (undead) XP 300
Initiative +1 Senses Perception +2; darkvision
HP 116; Bloodied 58
Regeneration 5
AC 17; Fortitude 19, Reflex 15, Will 16
Immune disease, poison; Resist 10 necrotic; Vulnerable fire, radiant; if the zombie tank takes fire or radiant damage, its regeneration does not function on its next turn.
Saving Throws +2
Speed 7 squares, climb 4
Action Points 1
M Slam (Standard; at-will)
Melee 1; +6 vs AC; 2d6 + 3 damage, and the target is pushed 3 squares and knocked prone.
m Smash (against a prone target; sandard; at-will)
Melee 1; +6 vs AC; 3d8 + 3 damage.
a Rock Throw (Standard; recharge 3456)
Area burst 1 within 10 squares; 1d6 + 3 damage, and the target is pushed 1 square and knocked prone.
Swat (when an enemy moves adjacent to the zombie tank; immediate reaction; at-will)
The zombie tank makes a melee basic attack against the triggering enemy.
Alignment Unaligned Languages --
Skills Athletics +9
Str 17 (+4) Dex 11 (+1) Wis 13 (+2)
Con 18 (+5) Int 3 (-3) Cha 3 (-3)

Ah, the tank. The players appropriately grew terrified when the tank arrived, although due to its lack of entourage it failed to live up to expectations. And while its Slam and Swat abilities proved accurately powerful, I never got a chance to use Smash, which would have a significant chance of knocking a level-appropriate character unconscious. Due to the interior setting I also never used Rock Throw, but only pansies use that anyway. Unfortunately I could figure out no way to included the tank's ability to kill everyone with one punted log, car, or forklift and stay balanced.