does every gym you join have a contract you have to sign?

Question

what if you only want to stay for like 4 months

Answer 1

Alot of them do, but most of them still offer a month to month fee too. You should be able to go and pay for a month, then when you want to, sign up for another month.

Answer 2

A good manager will give you a custom contract.
Obviously they want at least a year.
The longer the contract the better the monthly rate and the lower the initiation fee.
Try the gym you want first then try a small gym and see what happens.