Probably to prevent confusion with the American F-18C. Typically, an aircraft built for an allied nation has the letter appended AFTER the main designation. IE. an F-15J is basically an American F-15C Eagle, built for export to Japan. F4G Phantoms are operated by Germany.
'C' for Canada would create too much confusion if it were used in the usual location. Most aircraft types eventually get to a 'C' model.
Incorrect. Air Force F-4Gs started out as Navy F-4Es. The Navy also flew a small number of F-4Gs. The Luftwaffe flew F-4Fs and RF-4Es. The variant designator is not country specific.