obstruction or not
 

R1 on 3rd, R2 on 1st no outs. Defense puts a play on. R2 attempts to steal 2nd and catcher throws to F4 and draws F4 into path of R2 and a slight collision occurs with ball in flight. Based on rule 8-1-3e I see obstruction. When would we ignore and call this incidental contact?

What rule set are you playing by? In NFHS (and likely USA), it only becomes incidental contact when the ball is in the fielder's possession prior to the runner colliding with the fielder. A fielder risks an obstruction call when she gets into the runner's path while waiting for a throw or is in the act of catching that throw. But once she catches the ball, she's good to go.

In NCAA play, it becomes obstruction when F4 blocks the runner's path without the ball, even if she subsequently catches the throw before the runner runs into her.

The contact is irrelevant. Either a defender without possession of the ball impeded the runner or didn't. If it did, OBS; if not, play on