Offensive interference or nothing?
 

OBR, R1, batters hits grounder towards F4. As F4 is charging, the ball stops in the tall grass and F4 stops in the path of R1 and a collision occurs. The ball is 5 yards in front of F4 as R1 crashes into F4. Is this offensive interference or did F4 give up and is F4 not protected on this play?

Yes.

I vote B.
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Which makes it obstruction

Why did F4 stop?

Per the OP the ball stopped in the tall grass. The only reasonable conclusion is that F4 then gave up on it. Now not making a play on it - now not protected - now obstruction.

I disagree that is the only reasonable conclusion. He may have stopped to avoid a collision with R1.

That is still obstruction.

He was a significant distance from an unmoving ball. There was no play being made on the ball by then.

Disagree. He can still be the protected fielder.

Could be either, depending on whether the umpire judges the fielder to be protected and in the act of fielding. Here's how J/R defines it, and whil I agree it's correct, I'm not sure it's complete:

(a) A fielder is "trying to field" (or "in the act of fielding") a ball when
(i) he is positioning himself for the purpose of trying to glove
a rapidly approaching ball, or
(ii) he is actually gloving the ball, or has gloved the ball and,
without having to take steps, is trying to gain possession of
the ball, or
(iii) he is actually throwing the ball, or completing his throwing
motion after throwing the ball ("following through").