In general, I don't see a balance problem with allowing higher level potions so long as the recommended pricing and scaling is kept. There are many potions that clearly shouldn't be a problem at all, including some useful ones like Cure Critical Wounds (4th level), Stoneskin (4th level) or True Seeing (5th).
On the other hand, some spells probably shouldn't be allowed as potions, especially ones like Miracle, Wish or Time Stop. I think most divination spells would probably be ok no matter the level - "Drinking this mixture will allow you to see all things, past, present and future!"
The idea of having another feat to brew potions for levels 4 thru 9 is workable, but I'm not sure it's really necessary either.
The same argument applies to wands. However, I can see where certain high level spells might get a bit out of control if the user can fire them off round after round from a wand. Some common sense on the party of the DM would be needed before allowing certain wands into his/her campaign.