[tcblue13]

The NFHS rule says
"a runner is entitled to advance without liability to be put out when:
b. A fielder not in possession of the ball, not in the act of fielding a bated ball, or not about to receive a thrown ball impedes the progress of a runner or batter-runner who is leagally running the bases." 8-4-3b

I think it depends on how close the fielder is to receiving the ball. If the ball is still in the hands of the catcher OBS clearly

If the ball is reaching F4's glove in any close proximity to the actuial play no OBS since the she is about to receive a thrown ball. It appears that by NFHS rule she is entitled to block the base.

I would have to say it is a judgment call depending on how close the fielder is to catching the ball.

My question then becomes: What if the fielder about to receive the thrown ball blocks the base but then misses the catch preventing R1 from either advancing further or F4 can pick up the loose ball and apply the tag. Bang bang before R1 can react to the missed catch.