Here's a changelog of what's different/new according to Skell (forum topic post[forums.tripwireinteractive.com] or otherwise):

V4:
The changes were mostly balance and polish to improve the flow and enjoyment of the mod.

V3(taken from forum topic post as of 11-3-19):
Map:

Uses the fixed versions of specimens.
All doors are set to the enumerator ME_IgnoreWhenEnroach.
Moved the 2nd room's lower chest so that its spawned uber could look normal when spawning away from under the overpass.
Some graphical fixes (changed a couple of projectors out for something better).
Fixed server crashes on map change.
Rebalanced difficulty scaling for game difficulty and player number.
Rebalanced zed squads.
Fixed some issues with players not respawning (not sure).


Zeds:

General:
All zeds now have much more responsive hitboxes.
Animation error spam has been fixed.
All zeds now have footstep sounds.
Fixed all socket names.

Ventriloquist Dummy:
Increased Health to 325 (from 130).
Increased HeadHealth to 65 (from 25).

Uber Ventriloquist Dummy:
Zap Threshold is now 5 times higher (from 0.25 to 1.5).
Added difficulty scaling to this so that it goes up to 3.0 on Hell on Earth.

Increased Health to 2048 (from 1500).
Increased HeadHealth to 1024 (from 700).

Ventriloquist Knifethrower:
Increased Health to 325 (from 130).
Increased HeadHealth to 65 (from 25).
All burn resistance code overwritten to no longer receive a damage reduction.
Removed exploit with knives where they could be picked up as crossbow bolts.
Knives have been completely reworked to act more like knives.

Pinwheel:
Increased Health to 720 (from 525).
Increased HeadHealth to 550 (from 525).
Reduced melee damage by 1/3 (from 12 to 8).

Babydoll:
Increased Health to 98 (from 70).
Decreased HeadHealth to 50 (from 70).

Jack (Boss):
Collision cylinder (hitbox) now toggles depending on whether or not he's hiding.
ZED Guns (MKI and MKII) now deal damage to Jack.
Explosive weapons now also use their impact damage.
Time between attacks increased.
Time reduction per hide phase reduced.
Throws 1 extra bomb per throw time.
Damage increased to 200 (from 100).
Radius increased to 300 (from 220).
Speed decreased to 1750(from 2000).
Explode timer increased to 4.0 (from 2.0).

Throws 6 bombs when going to hide (instead of 10).
Hiding bombs act differently from regular ones
Damage increased to 200 (from 100).
Radius increased to 300 (from 220).
Speed decreased to 800 (from 2000).
Explode timer increased to 6.0 (from 2.0).

Shoots 3 saw blades (instead of 5 knives):
Damage reduced to 10 (from 15).

V2[forums.tripwireinteractive.com]:
Zeds:

General:
All hitboxes have been readjusted to work properly. Go out there and pop some heads!

Knifethower:
Fixed bug where players could pickup knives as crossbow ammo.

I'm not sure what was done in V1 and thread seems to be overritten, but will update this if I get a ahold of that information for completeness....