It's a big one!
4.0.0 pre-release users must reset language.yml!
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[ Added] Limited rotten flesh hearts level that users can get to 30 (prevents tons of hearts that will break game fun)
[ Added] Zombie idle processor to save server load and prevent lag! Game will halt spawning zombies for x seconds (wave / 15 rounded down) in wave to save performance a bit without spawning tons of hordes at once
[ Added] Zombie spawn limiter, maximum of 750 zombies can be spawned in wave now, any amount higher will be set to 750 and zombies will receive extra hearts as waves increments. Health is based on (zombies this wave - 750 / 15 rounded up)
[ Added] Game will now clean rotten fleshes from ground spawned after game ends in arena world in radius of 150 blocks near arena start location
[ Added] Tab completer for some command arguments
[ Added] Legacy data converter for YAML files, data saved in YAML was broken since ~3.11.x, user data will migrate safely and player stats will be updated
[ Added] Italian locale
[ Added] Added useSSL=false argument in mysql.yml database to avoid not harmful warnings in console (you can add it yourself to disable them too)
[ Added] /vda clear wolf argument to clear spawned wolves in arena
[ Added] Totally new entity upgrade system implemented! Shift + click to open it for golems and wolves!!!
[ Added] Wave limiter - limit how many waves game can play
[ Added] /vda setlevel and /vda addlevel commands were added
[ Added] JavaScript engine into rewards.yml secion, you can execute java code from rewards now
[ Added] Over 8 other additions
[ Changed] Whole code performance went up! Just check out your timings this time!
[ Changed] Buffed and nerfed cleaner kit ability, he can now use it every 60 seconds but it will clean only spawned zombies
[ Changed] Zombies won't spam lava particles on death via command/powerup/cleaner kit, now if there will be over 70 spawned zombies every third will only spawn particle to prevent client lag
[ Changed] Whole API is now improved!
[ Changed] Over 7 other changes
[ Fixed] Arena signs weren't properly saved via setup menu (4.0.0 pre-releases bug)
[ Fixed] /vda forcestart didn't start the game when there weren't enough players to start
[ Fixed] Worlds were generated if arenas.yml contains their names (reported by @Blackyvk)
[ Fixed] Worker kit wasn't displayed on 1.11-1.12 (reported by @Blackyvk)
[ Fixed] Doors couldn't be placed in game nor zombies could break them on 1.11-1.12 (reported by @Blackyvk)
[ Fixed] Commands without permissions weren't working (4.0.0 pre-releases issue)
[ Fixed] Setup menu wasn't working (4.0.0 pre-releases issue)
[ Fixed] Now not localized Spawn Golem/Wolf items will work with locales and in shops
[ Fixed] Over 11 other fixes
[ Deleted] Villagers won't stay in arena after the game ends anymore

More info about changes here and here
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