Yep, there's nothing wrong with using a SF at the 4. I had to do that with my Post team in Rupp this season due to losing out on a recruit and my SF playing PF averaged nearly 15 PPG and 8 RPG.
Hell, many, many seasons ago in Tark at Ole Miss (remember, this is D1), I got away with using a SG as a PF for about half the season. Of course, he was maxed out in nearly everything (this was before potential. Dude ended up with a total rating north of 1000). His only weakness, and it was a big one since I ran a Zone, was his shot blocking, but even with this SG playing PF, I was still able to BS my way into the Sweet Sixteen.
For the newer coaches in attendance, try not to get caught up in total ratings or what positions WIS designates a player as. If he's a PG who has the right ratings to play SF, go ahead and put him there. Play to your team's strengths and ratings, not their "designated" positions.