4 and 10 at K-20. K1 punts the ball. After the ball is clearly beyond the expaned neutral zone but while it is still in flight, R2 pushed K3 in the back at the K-28 yard line. R4 catches the punt at the K-45 and, because he is trying to get around the pursuit, he runs sideways and backwards and is downed at the R-45. Where is this PSK foul enforced from?

using the what ever hurts em the most principle, I would say the R 45....

The PSK spot is the K45 (end of the kick). Because the foul occurred in advance of this spot, enforce from the PSK spot. This results in no net loss of yardage. If R had retreated further after the kick ended, K may choose to decline the penalty.

The PSK enforcement spot is where the kick ended, the K-45. That is where the bean bag has been dropped.

The Spot of the foul is the K-28. Use the principle all but one and step of 10 yards from the K-45 which takes you to the R-45 which just so happens to be where the play ended.
So in this case it really doesn't matter.

Thanks everyone. That's what I was thinking.

Is this the same as the NCAA rule?

PSK uses the ABO principle. The end of the kick spot is the basic spot. If the foul is by the offense, behind the basic spot, penalize from the spot of the foul.