This little jewel has popped up before, but I finally found a work around.
If you are on a WiFi network then the phone should not be using 4G (or EDGE). If you see the WiFi and the 4G icons in the notification bar simultaneously then something is wrong. When this happens to me I can see that the phone is, in fact, only on 4G even though it says, "I'm using WiFi". This can be a problem if you don't want the phone sucking up your data plan bytes.
The work around: Go to Setup > Wireless and network > Mobile networks and toggle the Use packet data item off and then on. The 4G icon will go away, come back real quick, then go off for good. If you check it should be on WiFi exclusively.