The 4GC or 4-Jet was/is a square bore carburetor, square bore is defined as all four barrels/bores/venturis being the same diameter. The 4GC is basically the joining of two 2G's. The common notion of the Quadra-Jet is a spread bore carburetor, the primary venturis/bores/ barrels are considerably smaller than the secondary bores, these small primaries have the job of inducing fuel on start-up, idle, off-idle and mid/transition fuel delivery. The small primaries of the Q-jet have high velocity vacuum and do a good job of fuel atomization for the for-mentioned duties. The very large secondary bores of the spread-bore carburetor are designed to deliver a large volume of fuel and air when high horsepower is demanded. The Q-jet had a good 20+ year run and was still in use just prior to modern electronic fuel injection, it was GM/Rochester's best all-around carburetor for basically every GM vehicle with a V8. If you are thinking about experimenting with a 4 bbl., the Q-jet would be the easiest to acquire vs. the 4G, and would probably offer better low rpm benefits due to the small primaries, just stay out of the "go peddle" and secondaries.
