Yes, you can still ask the seller to pay closing costs but they don't have to agree. Also, the lender doing your loan may have restrictions on it so make sure to check with them.
Don't forget to use up the funds that are agreed to. If you ask for, and the seller agrees to, $4k back in closing costs and the estimate for your closing costs comes out to $2800 then you should look into buying down the rate for your loan to use the balance of the $4k.
Good luck with your purchase!