I sympathise! My son will be five next month and we had exactly the same problem last year. He finally did a poo on the potty about a week before his fourth birthday. I too tried everything, but being a wilful little so and so, the only thing that really worked was bribery of sorts. He got a sparkly Dora sticker to stick on the potty for every poo he did in it, and the promise of a Spiderman toy if he did all his poos for a week on it! Als o the fact that his younger brother who wasn't yet two was showiing an interest in the potty spurred him on.....he didn't like the idea that baby brother was going to be a big boy before him!
Toilet training seems to be so easy with some kids and others, for whatever reason, have a tough time of it. He's still not dry at night, by the way, but we are persevering with the 'no nappies for big boys' and there is absolutely no pressure. We just accept that some nights we get woken up by cries of 'Mummy! I've done a wee!' and get on with life :) Lots of prasie for success and justtake accidents in your stride is the best policy, I think.