RE: channel 4
In most cases where Georgia counties use the VHF-hi band, channel 4 is direct car to car communications on 154.905. Channel 3 in most VHF hi counties is where the base transmits on 154.905 and the mobiles call back on 154.935. Base to base communications typically happen on 154.935, or sometimes 155.370 . Of course this can vary with these two frewquency pairs in some places. For example, the City of Fort Oglethorpe uses the Chattanooga 800MHz trunked system for most ops, but they also use "National" 155.475 direct simplex to chit chat.
Hope this helps,
BG..