My betta's tank hasn't been cleaned in like four weeks but it's not foggy and doesn't look dirty is it ok?

I'd be concerned about Nitrates building up. They are toxic to fish in high doses, but we can't see them in the water column. Eventually, if you don't clean it like you are, you are going to have an algae bloom and the fish may suffer from Nitrate poisoning. Try to do partial water changes of 20-50% every week to keep things like Nitrate in check.