Yes you can, but it may be more trouble than just forking over the extra 400.. keep your receipts and get a statement from your landlord that you paid in full, this would be taken to civil court, you can threaten the roomate to play or be sued, that might press him.

If he is not on the lease, and if you do not have a sublet lease between you (the renters named on the lease) and him, then there is no legally binding agreement for him to pay you, or anybody else. The lawyer and court fees to sue him would probably cost you more than you feel he owes you.

The landlord needs to do the suing. You can not legally collect rent for property you do not own unless your landlord gave you authority to sub-lease.

You can take him to small claims, but since you had no written lease proving that his rent was monthly ... he will probably claim that he rented on a weekly basis and does not owe you for the balance of the month.

Since you did not have a lease with him, he is considered a month-to-month tenant. Laws vary from state to state, but here if he pays rent for more than 2 months in a row, that establishes tenancy. If he paid in cash, you will have a hard time proving it in court, but if he paid with checks you can show the checks as proof of tenancy. Since he paid you in weekly installments then he is only required to give you 1 week notice to move out. So technically, you are only out $200 legally. Again, laws vary from state to state, but here tenants are only required to give as much notice as there are days between rent payments. So if you pay rent on a monthly basis, you have to give 30 days notice. If you pay weekly, you only have to give 7 days notice.

So you could sue for the $200, but it probably isn't worth it. You would have to prove that he lived there for at least a few months, that he paid you rent and that he left without giving notice.