The 4 month regression is related to sleep becoming more cyclical, so babies who were able to sleep 1.5-2hr in a row start waking after 40-45mins and need to be resettled back to sleep until they learn to get through that transition of light sleep.
What you're describing is that he's waking 20mins after you've put him down and that screams discomfort, not regression.
Some refluxers do have a period of time when the symptoms are worse and from what you've said here and in other posts, I'd say for your little guy, that's from 2am onwards and by the time that first nap comes around he's so exhausted he crashes and sleeps. Then once he's just that little bit more rested, he struggles to stay asleep for the next nap and the whole cycle starts again.
I'm sorry you're in this situation, sweetie, I really am. There is no type or amount of sleep training that is going to make your little guy sleep better til that reflux is under control. AP however you have to, to keep yourself sane and rested enough to keep going.
Hugs xx