No, you don't need to piss every ten fucking minutes AAAARGH.

[BertieBotts]

Any ideas what might be the cause of frequent weeing in a 4 year old? I've taken him to the GP twice to have his urine tested and they say there's nothing wrong with him, but it's driving me mad and especially seems to happen after bedtime, he gets really distressed if I won't let him go, but it goes on for hours and he ends up falling asleep at about 9, 10pm.

He can happily go for hours during the day when distracted by interesting things without weeing, and if I can get him to go to sleep then he's dry in the night for 12 hours or more, and has been since he was about 2 - well before he was potty trained in the day!

My DD had this at around the same age - sometimes it felt like i was spending half my day in public toilets when out and about. very frustrating and sometimes hard to keep your cool when heading for the toilets for the umpteenth time, which obviously doesn't help but we're all human.
She was tested for a UTI and it was clear. Deep down i knew it was anxiety about starting school and sure enough, when she had her first taster session at school it all stopped immediately. Not much help to you OP but just to let you know anxiety can have a massive impact on urination. We had tried talking to her about her worries, lots of extra reassurance etc but it was only when she saw for herself and put her fears to bed that it just stopped. Even now, actually, if she's anxious about something I'll notice she'll wee more frequently. It's her thing (and as a parent quite a useful signal that it's time for some quiet time for a little chat and see what's on her mind...)
Hope it continues to get better for you.

you could also try having a potty in his room with this tape around it, so he can see where it is in the dark.