I feel your pain!!!
Honestly it sounds like you are describing our son he is the EXACT same. He is almost 5 mths now ( I had put a post on this forum about it already). It is so hard isn't it. I am beginning to wonder is it just what happens around this age? Or maybe that is just me trying to accept it rather than put it right, as anything we do seems to make very little difference. Our little guy sleeps ok till about 2 but from then on it is anyones guess, we are in with him 2,3,4,5,6,7 times during the night trying to get him back to sleep ( and he can do it himself as we have heard him do it on the monitor in the early evening, but just not late at night). We use pat ssh to get him back to sleep, but sometimes it takes ages. Seems to be he cant resettle himself. As to why he wakes so often, I have no idea. He takes short naps too ( 45 mins, only very occasionally longer). At the weekend he did actually extend his naps, but it made no difference to the night on either occasion. Maybe it is just the way they are!
Is your LO in their own room yet? We moved ours a few weeks ago just to see. Hasn't seemed to make any difference to us, but some people really say it does, so that might be something to try. I guess other things you are probably doing, like having a consistent bedtime routine etc etc. Are they getting enough calories in during the day to see them through the night??Not sure if I have anything else to offer you but just to say you are not alone. Sometimes it is so hard when everyone else has a baby who sleeps well, it really is exhausting. I guess eventually they will just do it, all in their own good time.