I cannot help with the reflux, however it is a bit young to start with sleep training. The earliest recommended age is 6 - 9 months. At 4 months he still needs the security of knowing you are around for him.
But you can start preparing him for sleep training by making "sleepy time associations". Get a sleepy time toy eg blankey, teddy, dummy taglet.
Wear it under your shirt, close to your skin so that it picks up your scent. Then get him to hold it (you may need to pop it under his arm in the beginning) while you feed him and rock to sleep.
You can also get a sleepy time routine in place eg. bath, bottle in darkened room, story / song/ rock and then bed.
My routine with both boys in the beginning was bath, bottle (sang to them while feeding - same song everynight), bottle out, then I would stroke cheek, saying "sleeping time darling" occassionally and sing the sleepy song until they were asleep.
Then with Liam I would wait until he was asleep and put him into bed, then after a week or 2, I would catch him just before he fell asleep (the breathing changes), then pop him in his cot.
I can now read his story and put him down wide awake - no crying was ever involved. Okay i tried sleep therapy with him, cos it worked a bomb with Brandon, but Liam would projectile vomit. This system did it for us and was my lifesaver - started just before he turned 12 months and within 4 months we had it waxed.