@Tudor648 I basically did this in one go with EXTREME luck. I saw how horrible my team was in challenge 4... wait this is about challenge 4? XD I thought you were talking about 5...
Okay then nope, did this in 2/3 goes. I chose peashooter as well. Basically what I did, as soon as I was revived, ran down to the docks, got in peagatling and started buzzing. Usually died, but at least took 2 with me and wounded 1... only to see that 2 of my AI teammates died by the same character I died to...
I didn't have any special upgrades. I was a lvl 1 pea basically. Did the vampiric (always have that one), I think 'fearless' (gain health when near multiple enemies) and last one was probably the one that reduced wait time when sprinting. Do that tactic and you'll win... hopefully...
Now for the last battle chest... as soon as you see disco... start your gatling and take him out. Don't know if it's a bug or whatever, but it felt as if electric slides became less aggressive whenever you took out a Disco. And Disco also kills way more, so with any of them gone, means less dying AI teammates...
If you don't see any Disco's there's probably 2 Electric slides trying to double team your teammates. Gatling them and save your bomb for the Disco. won it with 50-46... while initially I was 5-12...