TOTALLY NORMAL! :)
Cari,
I have a 4.5 month old and she started sleeping on her tummy at 3 months. There was no stopping her. We used a sleep positioner up until she started wiggling her way out of it (even being swaddled). So I recommend NOT using a sleep positioner as that may become more of a safety hazard than a solution.
Once babies can roll over and move their heads from side to side the risk of SIDS goes down dramatically. Continue keeping blankies, pillows and stuffed animals out of the crib though. I got a mesh crib bumper in place of my regular one so IF my baby gets mashed up against the side, I know her breathing won't be obstructed.
There is no way to stop a tummy sleeper from sleeping on their tummy. It is what's comforting to her and trying to change that by flipping her to her back in the middle of the night could cause her to wake up and just get frustrated.
I know you are concerned about SIDS, but believe me, she is going to be alright!