This happened with my second one. He went from being a great sleeper to a very restless one. I couldn't really swaddle him because he too was big and strong and seemed to get frustrated.
One of the things that worked for me I found out by accident. I laid him down on his tummy to see if he disliked it as much as his sister but to my surprise, within seconds of laying him down, he put his thuimb in his mouth and fell asleep. I know this is not recommended but I kept my eye on him and he was fine. He had not really sucked his thumb before and had definitely never fallen asleep so quickly or easily. This worked like a charm.
The other thing that I noticed that helped was he wasn't getting full enough. I breastfed him and pumped and stored for when I went back to work. After I breastfed him I would give him a couple more ounces of stored breastmilk and he would fall asleep more quickly and sleep for a longer time.
I think 4 months is a little too young to sleep train. I personally don't believe in it at all. It takes time for them to learn to soothe and get in a routine that is comfortable for them. Also, if you are tense or stressed (which I know new babies kind of have that effect :-))he will pick up on your tension.
I practiced attachment parenting with both of mine and after the initial adjustment period they became great sleepers and still are to this day.
Whatever you choose, I wish you luck...and sleep :-)