R.,
Going from the gut here - use a swing.
Are you still nursing? He's up 2-3 times a night - he's only 4 months old, he is probably hungry. It is faster to nurse him (or a bottle - have a mini crock pot with water for a fast warm up) than worry about him being up.
All babies are different. Mine was up 4-5 times a night at that time. He gradually got down to 2-3 when he was 13 months. He's 17 now and down to 0-2.
Patience is his best friend from you right now. I know you want your sleep, but figuring out what he needs is the best thing you can do. You've done a great job so far.
I'm guessing he may not need the swaddling anymore, just cuddle him before he drifts off.
Boppy is not a good thing, but I just read back and it is a good thing you mention it. He probably has reflux, at least a little bit. Sleeping in his carseat would probably be great if you don't have a swing. Side to side works best at this age, but there are some that switch front-to-back and side-to-side. If he is your only boy, a used one will do just fine - but if you are planning on more - you may wish to buy new so it will last. (the motor goes eventually, like a lot of things :( :)
Also, if you are looking at this - add another feeding during the day, possibly in the early AM. It might give him a feeling of fullness that either he can look forward to (daylight = more food!:) or will start him off not feeling hungry during the day.
Good luck, these are not bad habits and let me know how you do - he's 4 months old and he'll do fine!
M.