This actually just happened to me a couple weeks ago at a training session! I put up 4 fingers to show the R1, but he was too focused on the play to see my help. My observer told me, in that situation, to do the following:
1) Move to the side of the net where the 4 hits occurred and STAY THERE. Stop moving with the play.
2) Hold up the 4 fingers, chest high and OUT away from your chest. You are no longer being subtle.
3) If the R1 still doesn't see your signal at the end of the rally, blow your whistle while still holding up the 4 fingers.
At this point, everybody in the gym knows that you think there were 4 hits, so you better be 100% sure!
The R1 then can decide to ignore your help and take the rash of manure that will surely follow, or s/he can have a "change of mind" and award the point to the team that didn't commit the initial fault.
I don't think this is a replay situation.