I went through the same thing right before 4 months. It was a tough couple days without the swaddle because he thrashed around a lot, but over time he would roll over on his tummy and sleep a lot better. Two things I learned from my ped:
1. It's okay for them to roll on their tummy if they can do a push up, which I'm sure your lady can do by now.
2. Still always put them to sleep on their back and let them do the rolling over themselves. (I don't know if my ped is just being super cautious but we'll follow this advice through the first year and check on him probably a little too much :) .) From what I've read, SIDS risk peaks at 2 - 4 months.
Also, a halo sleep sack/wearable blanket is ideal while they're in this developmental stage, as you don't have to worry about a blanket getting all over the place or over their mouth while they squirm around. We used it every night in the winter.
The bonus is once they're sleeping on their tummy, they do tend to sleep a lot better, if they're the kind to flail around a lot like my guy.
Good luck!