In addition to Panzer Tracts No. 4, here are a few more that might help.
Panzerkampwagen IV and Its Variants 1935-1945 (The Spielberger German Armor and Military Vehicle Series)
Panzer IV & Its Variants (The Spielberger German Armor & Military Vehicles, Vol IV)
Panzerkampfwagen IV Ausf.G, H and J 1942–45
Panzer IV: The Panzerkampfwagen IV Medium Tank, 1939-1945 - Armor Specials series
Panzer IV: 1939-1945
The first Ausf. G's, and redesignated Ausf. F2's, had the original L/43 Main Gun. In April 1943, this was replaced with the L/48 Main Gun. Therefore, a March 1942-April 1943 production vehicle will have the L/43 Gun. An April-June 1943 production vehicle will have the L/48 Gun.