Well my DS1 was 4 in September. He was potty trained 1 month before his third birthday (although no.2's have always been a problem). In September we decided to try him at night LMAO - it last for one month (mainly because HE kept saying he didn't want to wear a nappy).
However despite, limiting drinks before bed, getting him up in the night to wee and lots of encouragment we had 5 dry nights in 4 weeks. He started getting stressed because he'd 'had an accident' - I got stresesed because I had to wash his full bed linen (he wouldn't get up and tell us when it happened just stay there until he got up - and he's also a great wriggler so it used to 'spread', duvet cover, pillowcase and bottom sheet) and we agree that he'd go back into nappies for a while.
It's quite a squeeze fitting him into the nappies but I'm not going to rush him to try and get him out - so what if he's 5 years old when it happens.