What are you calling it? iPhone 4 Antenna cable fastener? Contact clip? Little thing under the battery connector screw? Whatever you call it, we have it, and you should be stocking at least one of them. This is the fastener that holds the iPhone 4 antenna cable and also makes contact with the back cover. It sits above the battery connector and attaches with the same screw that holds the battery connector in place.
eTech Parts Blog 
It’s part of the GPS antenna. Leave it off and try using a GPS app, you’ll get lost in about two minutes!