I would imagine that ranged attacks take the penalty associated with charging according to their movement distance. If the mount only moves up to its speed on the charge, then the rider takes no penalty on its ranged attacks, if the mount moves more than its speed, up to double its speed, rider takes a -4, if somehow the mount can move more than double its speed on a charge, the rider takes a -8.
This isn't anywhere RAW, but I'm drawing my opinion based on precedent from the lines in mounted combat about ranged attacks on a "quickly moving" mount:
"You can use ranged weapons while your mount is taking a double move, but at a –4 penalty on the attack roll. You can use ranged weapons while your mount is running (quadruple speed) at a –8 penalty. In either case, you make the attack roll when your mount has completed half its movement. You can make a full attack with a ranged weapon while your mount is moving. Likewise, you can take move actions normally."
And I'm drawing from the charge rules, where there is some indication that it is "easier" to charge only up to your speed, as opposed to double your speed, since if you only move up to your speed you can also draw a weapon as part of the charge (but cannot draw a weapon as part of a charge where you move any more than your speed (without quickdraw)). The quote below is taken from the "movement during a charge" rules:
"If you move a distance equal to your speed or less, you can also draw a weapon during a charge attack if your base attack bonus is at least +1."
This obviously has nothing to do with ranged attacks, but personally I think these two together serve as a standard for balancing for a good way to rule it. Obviously if there's any official ruling on this that I just overlooked, completely disregard what I've said.