Newbies first, then veterans can correct them.
Bases loaded, 2 outs, full count, R1 and R2 off as the ball crosses the plate. R3 not so bright. Pop up near 2nd base. R1 scores easily. R2 scores before the ball comes down. R3, seeing the popup, gives up and is standing about 15 feet off of first base.
F4 fails to catch the popup. After the ball hits the ground, F4 chases R3 back toward first, where BR has already rounded the bag, causing R3 to run (in reverse) past BR. R3 gets all the way back to the base and is tagged (BR is about where F4 started the play when the tag is made).
How many runs score and why?
edit to correct R1/3 error and technicality on timing of when R1 and R2 left)