Luckily, I didn't have too much of an issue on this with my son, but I do agree with Nikki~
Make sure you keep just a nightlight on in his room for nursing or changing - just enough light to see by. (Any brighter and it makes the brain feel like it's time to wake. Ever notice that if you use the light in the bathroom yourself?)
He could be wanting to nurse for comfort - he is really hungry or not? Some breast-fed babies will do this. My son did, but he was also a grazer... still is at five when he eats! :P
Make sure you are sleeping during the day while your son naps - forget the house right now. You NEED to rest your body, too. Many moms forget this or try to do it all, and you really cannot. I tried to, and finally I gave up as it was too much. I found I was getting depressed - not with PPD, but just upset to where I felt I was useless at not getting my regular stuff done. I finally told myself "This is my baby - he needs me at my best, so I need to rest." and it is more fun to snuggle with him anyway! So that is how I did... I'd lay on the couch with him and both of us would nap - and it would be longer that way.
Sleep is needed not just for the immune system, but also to help keep your Breast Milk supply going.
Try to engage him a bit in the day - tummy time and play mats.
But keep his room at bedtime / nighttime dark. You may have to keep giving him a pacifier (if you have one) to get the point across - or realize it may be a stage. (I do have a friend where her son wakes at the same time. )
Make sure he nurses on both sides...don't let him 'nap' as my son would do that on me, so he fills up.
If he still wakes at 4am - you may want to feed him a 'snack' as nursing babies do get hungry faster than formula feed babies, and the nearness may make him sleepy again. Just make sure the lights are still off and you are very, very quiet.
Worse comes to worse - can he stay in the bassinet safely or does he have a crib where he can 'play' while you rest? You may be teaching him to have 'quiet time' early. (Obviously, you can't sleep in too long~ LOL - but you can rest for a bit longer.)