Try 'iPhone 4 plastic case' on eBay
You can get clear ones, or ones with funky coloured backs, if you fancy giving your phone a bit of a change.
Theyare all pretty cheap (99p-£2.99) and last really well. They give good protection. With out bring to bulky.
Nor to visible, infact a lot of the time you can't really see there's even a case on it