No limits on team size, but we do recommend splitting teams up if they reach 6 or more members. A team of 4 is absolutely fine.
As for multiple users, you can follow these instructions: https://itch.io/t/433489/can-there-be-a-game-by-multiple-people
You could also just use a singular itch.io account and credit all people in the description. The choice is up to you.