To get Chopper's 4th costume, you must have Usopp's story mode to the map area called Drum Kingdom Sea Area. Ok, go there and select Pepper Village. You fight Chopper (In the new costume he's wearing); beware, he's lv 60, so make sure you have a character that's lv 70 or higher with a high attack stat (Chopper has great Defense), I reccommend Sanji for this. There's a bonus to this, you have to kill Chopper in less than 20 secs. so be quick about attacking and pay less attention to the items. Some items do alota damage though, like the fire ones, try burning Chopper (lol).
After you defeated him. It announces that you unlocked Chopper's costume. Isn't he cute in it lol Well, see ya later.