Hi! Are there any restrictions on how many people participate in project? Like can group of 4 wirk on a game? And if yes, shoud we just put our names or links in game and about the game page, or do we have to do smth else?

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.