500 vs 600

Im really not sure what type of edelbrock it is. But I don't think it's a thunder series.

I just have a performer 500cfm. Am I correct that when driving around I am only using 2-barrels until I punch the gas to the floor? Even if crusing down the highway at 65mph I am not operating on all 4-barrels? Seems like I really can't feel it when the other 2-barrels open up. Maybe thats how edelbrocks are built.