hello, if we are a team of four, all the members need to register to the jam or only one?

Ideally, all of you will register for the jam so we can have a better idea of how many are involved.

But technically, only one person needs to register in order to submit the game - any person who has admin privileges on the game page.

Make sure you add team members on the game edit page by going to More -> Admins and adding all other contributors.