Here's your something positive: IT GETS BETTER. 4 month sleep regression is a bitch. My baby was the world's worst sleeper at that age. Seriously. She woke up hourly until about 11 weeks. Then she started sleeping in 7-9 hour stretches. I thought, "Great, things are improving". That lasted... Two. Weeks. Then the 4 month sleep regression happened and we were back to constant wakings, I'd guess every 90 minutes to 2 hours (Seriously. I have a thread on it!).
My LO is now almost 9 months old and she sleeps through the night! The night before last she slept 13 hours straight. I'm not even kidding. I'm well aware that she may go through another regression - There is commonly one at this age, but we haven't hit it yet. But the difference now is that I know it will pass. I guess logically at 4 months you "know" it, but you really haven't had the experience that things will get better yet so it's hard to believe it.
I think the previous poster does make good suggestions. How often is your LO awake between naps? Preventing overtiredness is key. You mentioned he goes down when he is tired, but as babies get older their sleepy signs are actually unreliable. Often when they're yawning, rubbing their eyes and getting fussy they are overtired and it's too late. I would aim to have him asleep within 90 minutes from when he last woke, especially if he's up that much at night - He needs to make up sleep during the day. That may mean starting the soothing process 60 minutes after he last woke. I really do think it will help, even if it means you're getting up 6 times rather than 10. Overtired babies wake more often.