If you've overstayed by about 3-4 days on a exempt visa, is it still possible to go to Immigration in Chaeng Wattana and extend it? I'm assuming it's the same as leaving the country and having to pay the overstay fine. Or will they say you can't get an extension due to overstay and will have to leave the country?

Thanks

They will probably grant the extension after you pay the overstay fine and fee. But they could just give you 7 days to leave the country.

There is really no excuse for doing it late since you can do it several days early at any immigration office.