I know the NCAA has a "fumble on 4ht down" rule but I am not sure how it applies to this situation.

A's ball 4th and 5, QB drops back to pass and is pressured. While scrambling toward the LOS he is hit behind the LOS and looses control off the ball:

A) and is caught by a running back 4 yards behind the end of the run and advances beyond the LTG.

B) and is recovered (grounded ball) by a running back 4 yards behind the end of the run and advances beyond the LTG.

I am pretty sure if the fumble goes foward, only the runner loosing the ball may recover & advance. And if another player recovers, it is returned to the end of the run. Correct? Does this apply to all fumbles in any direction on 4th down?

It applies to grounded fumbles in any direction. Sitch 1 is legal. 1st down! Sitch 2, it's B's ball at the spot of the fumble.

NCAA - In both, the ball is dead when possessed by the RB and it remains at that spot. It will be B's ball since LTG was not made. Whether the fumble is caught or recovered is immaterial.

I stand corrected. Was working on too many things at once.

Good thing we have those guys in the booth, huh?

No replay official needed - you worked that out on the field.

So the ball is spotted at the recovery if it is behind the spot of the fumble? If it is recovered beyond the spot of the fumble, it is returned to the spot of the fumble?

You got it. They will NOT gain by fumbling, whole intent of the rule.

Bingo.

If the play above were a grounded backward pass, I assume the RB could legally recover and advance?

Yep, backward pass can be caught or recovered and advanced by anyone (even an "ineligible")