At a first glance, to me it seems like he is not getting enough A time during the day. By about 4 months babies are creeping up to 2 full hours of awake time, and that may be even a bit more towards 2h15 by 5 months. He IS taking decent naps, but I suspect that if you can stretch out his A time then he will start getting naps a bit closer to two hours in. The waking up to play in his cot from 4 - 530 definitely seems to me to be a sign that he needs more awake time during the day ... he's not getting it so he just takes it at night when he can.
What I would do is start stretching his first awake time in the morning to 2 hours. Do it gently, not by filling it with lots of activity but nice quieter activity time. If he can handle that, and still do a nice long nap, then I would leave that one at 2 hours, but start pushing his second one to 2 hours. Then once he can do that and still give you a good long nap, start pushing that second A time to 2h10 min, or 2h15 min. His third A time is already two hours, that's great, and that last nap that you have at 315, you don't want to extend that one anyway. It's his catnap that he will start dropping over the next 6 - 8 weeks or so anyway, so keep that at 30 - 45 minutes.
A key thing to remember with it is that you should look at it as a gentle and slow plan to do over say, 10 or so days, rather than just one day deciding to keep him up longer. Otherwise he will get OT and you'll just have more problems! If you do it slowly and let him get used to the awake times, he should get used to sleeping a bit longer as well, and not needing the play time at 4am!
Is that making sense?
