Im not sure about iphone 4 but ive got a "gear 4" black rubber case for my older iphone, it covers the back/sides/top and bottom and just leaves the screen and home button etc uncovered. The back of the case(back of the phone) is like a tread patturn and really grippy, can even put it on my dashboard in the car and it wont move, thats how grippy it is

Maybe you can get that, or something like that for the iphone 4? have a look on amazon, they do loads of iphone cases and other accessories