Tenant moving out

There are four of us renting a house. One of the tenants is looking to move out. Are we required to give them their part of the security deposit back, and if so, do we have to cover that with more money out of the remaining tenants pockets?

Are you the LL? If not, you do not have their security deposit and this is an issue they need to take up with the LL. If the LL won't refund the part of the deposit (most won't), they need to either wait until everyone moves out to get the deposit back (assuming a joint agreement with all tenants signed on the same agreement) or get someone to replace themselves as a tenant (sublease or sign a new lease/addendum with the new tenant), and collect the security from that person. If they do this, they need to give a letter to the LL stating that they no longer have any interest in the deposit and that when vacated, the deposit will go to the new tenant.

Does this hold true for CA? I know we have some odd laws regarding rentals.

Why wouldn't it? Only the LL holds a security deposit. The other tenants do not hold it. The departing tenant needs to take this up with the LL, not with the other tenants. This is not a roommate issue.