If you are looking for a snap-on case for iPhone 4 or 4S, we would definitely suggest Acase case with dual protective layers. The case shell is made of silicone. It includes a polycarbonate layer as well. It is impact resistant and very protective against scratches which can appear especially on the back of the phone.
You are buying a perfect accessory for the price tag. Amazon customers have also given very high ratings for this item. Check reviews by clicking here.
The snap-on case is compatible with all available iPhone 4S models including Verizon, Sprint, AT&T for all storage options (16GB, 32GB, 64GB).
All ports, features and buttons are fully accessible. I liked the existance of silicone plugs for ports. It is effective protecting them against dust and dirt too. The case itself features anti-dust, anti fingerprint properties.
There are a few color choices besides black including red, white.