i am a 40 year old first time mother of a 7 month boy.
mine did the same with not burping except it was at night
right before he went to bed.. several times he cried out during the night and when i picked him up, he burped.
my cousin taught mme to feel the air in his back- if you rub around i promise you can find the spot he normally burps with.. otherwise, if you stand up with him, thats a better way for air to release. it works. not sure why..
i suggest you read babywise. i have followed it to a t.
my 7 month has slept through the night since week 7.
hes woken lately with a small cry but the book says let them for a few minutes and they go back to sleep.. you
need rest and a 4 month old is old enough to sleep 10
hours i promise.. good luck. L. b