venturi size is gonna make the diff, technically a 390 4 brl has smaller barrels all around, smaller venturi=faster air=better mix=more response/power till the engine can whip it up by itself.
so as long as the secondaries aren't opening too soon, the 4brl is gonna have the same or most likely better off the line.
you can tune around this and get both real close in response/performance, but in the end the 4brl has 40cfm more flowing through it.
the build will make the difference, some street performance builds won't eat all a 350 holley can give, it becomes this almost slowing down with more pedal and speeding up as you let out of it......
one thing that will also cause this is not opening the 2brl flange up to the matching bore size of the 2 brl carb....it just drops out and go's slower from lack of speed even at 3k rpm. the intake becomes the cork.