Hmmm... Let me ponder a bit. I'm not against this, but let me see what the RAW of the Basic/Expert books say.
EDIT: I'm back. I didn't agree with you, but then I chewed on it and now I do agree with you. You've made a convincing argument for stabilizing by standing in your stirrups. You've made an even stronger point by pointing out that we're playing Rules as Written, in which case even if I disagreed with your logic (I don't) I would begrudgingly concede because keeping this game Rules As Written is one of the main tenets of this campaign. I will strikethrough my post and redact that part of the OOC comment.
@dominion451 is right; there's no penalty for missile attacks on horseback.
EDIT 2: I'm chewing on this still... I can't let it go. I think we're going to go "no penalty" for horseback. But if this game was was leaning on rulings over rules, I think a very strong case could be made that shooting from horseback is a difficult thing (certainly more difficult that shooting from the ground) and if a PC had spent a significant amount of time in their life training to shoot arrows from horseback, they would probably have a pretty decent Dexterity modifier, which would mechanically offset any penalties incurred for unstable shooting.
But Rules As Written has been a foundational cornerstone for this game, and I can't find anything in the rules to support this, so I think I'm going to have to concede to @dominion451... You got me this time! *shakes finger...
EDIT 3: Grrr... STIRRUPS AREN'T STABLE!!!
Lets take a vote from everyone. My argument is shooting from stirrups is Unstabe Surfaces, and it would take a lot of training to actually do that. And if your character was training for that sort of thing, their Dexterity Modifier would reflect that, and offset the -4 penalty some. But I'm willing to concede to a vote if the rest of you disagree.