One question, was it a fumble or a backward pass?
Assuming it was a fumble recovered by a different player, on fourth down, only the player that fumbles the ball may recover and advance the ball. If another player recovers then the ball it is dead where recovered, if it behind the spot of the fumble the ball remains at the spot of recovery, if it is beyond the spot of the fumble it is returned to the spot of the fumble.
If the ball had been recovered by the fumbler, then there are no rules about advancing whether the ball is fumbled forward or backward. Another issue is if the ball is fumbled and it goes out of bounds.
If the ball is fumbled and goes out of bounds, whether it is 1st or 4th down if the ball is fumbled forward out of bounds then the ball is returned to the spot of the fumble, if it is fumbled backward out of bounds then the ball is placed at the out of bounds spot.
The reason I asked if it was a backward pass is that the provision doesn't cover muff's on 4th down. So if the QB takes the snap and pitches back to a RB/etc (backwards pass) who muffs it, anyone still can recover and advance since the ball wasn't fumbled. Same as with the snap to a QB from center, since that is also a backward pass.