He is also still however using nappies for number twos and frequently wets his pants while at daycare (which is only recently begun - a month or so - the first one was a fail, he bit kids and was very unhappy) and at home unless he's constantly and regularly cajoled to the bathroom - the only way we know how at the moment is to give him math challenges. If asked directly he will always claim not to need to go.
I'm not clear how the math challenges relate to using the bathroom - you sit there with him and do math while he poops? Or you reward him with a math challenge afterwards?
I don't think you should have to cajole a very intelligent 4 y.o. to use the toilet. If he's having consistent accidents then using the toilet periodically shouldn't be a choice - he should be required to go every two hours. You don't want him to think he has the right to make other people deal with his poop and pee because he's too engrossed in something and doesn't want to be interupted. Of course he isn't thinking this consciously! He is very young and will sometimes get absorbed in something and forget. But it's still a very real (and kinda gross) imposition on other people. When he is reminded to use the potty he should understand it is his responsiblility. He should want to avoid the consequences of not using the toilet.
I like the idea of sitting with him and doing math problems or reading him a book out loud while he poops. Seems like a good way to help him build positive associations.