K has 4th and 10 at K's 45.

K punts the ball.

While the ball is in flight or I guess on the ground past the exp neutral zone R4 blocks B5 in the back. The punt goes untouched in the end zone for a touchback.

1)R4 foul is at R's 30

2)R4 foul is at R's 15

What are the enforcement spots?

I am guessing in 1 it is from R's 20 1st and 10 @ R's 10

In 2 it is from R's 15 half the distance 1st and 10 @R's 7 1/2.

Any comments?

Hurt R the most theory???

The kick ends when the ball breaks the plane of the goal line. You have the enforcement spots right.
Not sure where you're going with the "hurt R the most theory" question.

Canadian Ruling
 

In Canada we do not have an expanded neutral zone, nor a touchback, so I will ignore those elements.

So we have a punt with a non-UR foul while the ball is in flight. The accepted foul is applied at PP. (8.2.4a)

The aforementioned "1)" and "2)" do not apply, as this is the PF, and has no bearing in Canadian football.

Since the PP was the EZ, the foul is applied from the 10 (8.4.4b). We're restricted, so we end up with B 1D/10 @ B-5.