Can specific armor and weapons be further enchanted?
i.e. Can I have a suit of +2 Rhino Hide?
or would the Rhino Hide (since it's special +2 Hide) count as +4 hide for it's cost?
If you can reverse engineer how they arrived at the price for the original item, you can work out a fair price for a better version. How doable that is depends on the item.
Rhino Hide is 5,165 gp so:
- Masterwork hide = 315 gp
- +2 bonus = 4,000 gp
That leaves 850 for the extra -1 penalty and 2d6 charge damage, which is obviously not being priced as a +1 equivalent, so it can probably be left alone. Meaning that Rhino Hide with a total +4 bonus should be 17,165 gp, or if you're improving existing armor an extra 12,000 gp.
Edit: Although like any custom magic item, you'd have to get this by your GM. In the case of just increasing the bonus, that's not usually a big deal.