I was interested to see this thread as my LO will be 5 months on sunday, and he consistently has problems with getting into a deep sleep after 4 am as well! He doesn't come fully awake, I just hear him constantly mantra crying as frequently as every 15 minutes, or as infrequently as every 30 minutes. It doesn't happen every night. We can go as often as a week where he only has one waking at 4 am. But 4 am seems to be the magic time!
When he has the really bad nights, I've gotten up to watch him, and I see that he's just jolting really hard and has trouble getting into the deep sleep. Sometimes I'll hold his legs down for him, and I can get him back into the deep sleep. But more often I'm just too darn tired myself to do that, so I play the paci game, where I go in, stick the paci in his mouth, and go back to bed and pray that's enough!
My guess is it's developmental. For instance, last night was horrid. Between 3:30 and 6:30 I got up 10 times to put the paci back in. What a dope I was. At any rate, I know the cause is because he's teething. It definitely wasn't pain cries (I know what THOSE sound like, believe me, because we're heard a lot of those the last few days!). If I'd been thinking straight, I would have held his legs down. Finally, at 6:30 I got my addled brains together and held his little leggies down for 10 minutes. That seemed to do the trick, because he then slept deeply until 7:30 - the longest stretch of sleep I'd gotten since 3:30 am. At that point, I got us both up since I didn't want us to get too off schedule today.
I'm glad to hear I'm not the only one who goes through this! I'm going to look in Dr. Weissbluth's book today to see if 4 am is a natural biorhythm time for wakings, because I'll betcha that combined with developmental changes is what makes them become light sleepers that time of night. I think if you can wake up enough to get them thru the jolts, that may get them back into the deep sleep. At least, now that I've heard all of your experiences, that's what I'm going to try from here on out!