it just isn't possible
+4 is a pretty powerful bow, it just wouldn't make sense for it to go higher.
If you wanted it any higher, this is what I would do.
The Japanese bows that use a special martial arts to learn how to use them, I'm not sure what its called. In history, a Japanese General with his bow (which took 12 men to string) sunk two boats with one shot.
I'd make the bow an exotic weapon, dealing 1d10 damage, and going up to +6 str bonus. It couldn't be used from a mount PERIOD, unless you're huge or something. There is no composite form, and it has a range increment of 150 or 170 or something. It costs a lot, and you have to make a str check to string it.
This seems a bit powerful, but anyone else have some ideas?