Old post, but incase someone comes across this with the same question....
Verizon requires antenna diversity from any phone connecting to its network. That means the phone must have more than one CDMA antenna. On the CDMA version of the iPhone 4 there are 4 "breaks" in the metal mid-frame. The top portion of the mid-frame is one CDMA antenna (receive only) and the bottom portion of the mid-frame is the other CDMA antenna (transmit and receive). These are the antennas used to connect to the carrier's network. The side of the mid-frame with the volume buttons is not part of the antenna system while the side opposite the volume buttons is part of the antenna system for GPS and WiFi.
The antenna contact the original poster is asking about is the ground for the top CDMA antenna. It has nothing to do with WiFi and there is no replacement part per se. If this is broken, eventually you will run into reception problems. You may not notice right away, but areas where you had low signal strength before breaking this connection will probably have no signal after breaking it.
There are two options for fixing this. The first is to solder it back on to the top section of the mid-frame. If that is not possible, then you will need to pick up a new mid-frame.
Hope that helps!