Well Tracy does suggest moving to a 4 hour schedule at four months -- some babies adjust better than others of course.
My personal experience has been that the closer I follow a consistent EASY, the easier it is to do everything. I don't mean that I clock watch strictly, but I do follow both the clock combined with my DD's cues. That way when she starts getting cranky, I know for sure why. When he starts getting cranky and you feed him, are you sure he isn't tired? A short 30 minute nap in the morning, especially if he's still a bit cranky, can be a symptom of OT.
As you transition into a 4 hour EASY, in a perfect world he'd have 2 hours awake (including eating, diapering, etc.) and 2 hours asleep.
When in your routine are you trying to put him down for his naps or to bed? After the first yawn or eye rubbing? Before? What's your routine just before sleep like? How much wind down time are you giving?
One thing I like about watching the clock to loosely plan our routine is that I can anticipate when the next nap will be coming, then get my DD ready 20 minutes before that... So by the time she's ready for her nap, she's already in bed and wound down.
How many bottles a day is he getting? I'm wondering if you can increase the formula to 7-8 oz (if he'll take it), that might help him hold off to a 4 hour EASY? Does he get solids at the same time as his bottle? My DD is breastfed but drinks 4 times a day (every four hours) but I don't give solids at the same time. I give her a feeding of milk and then an hour later her solids -- this works at our house to tank her up before her nap.
my post is a bit all over the place, but I want to try to address all the things you've talked about...
Since your LO is sleeping 9-10 hours at night, you already know that he has some capacity for self-soothing because when his short sleep cycle ends, he probably stirs a bit in the night and manages to get back to sleep on his own a lot... so hopefully you can use this good foundation to help teach him how to do it before a nap or before bedtime. If you put him down in his crib (?) a bit before he shows any signs of tiredness, what does he do?
Lastly -- if you try everything and nothing works, he is old enough for the sleep training method of pick up/put down. Do you have Tracy's book Solves Your Problems? She goes into great detail about how to do it, but there is also great info on this site! I haven't had to use that strategy too much, but from what I hear, if you do use it, pick a weekend or time when you're both home and don't have much going on because you'll need your sleep if he really puts up a fight!
Jana