I think the difference is an electric chokeI have the 1902 on my truck been a great carb for me. Where did you find a 1904 ? is new ? Edelbrock quit making them awhile back.
A Q jet will never give more cfm than the engine can use. Might have to tune the jets and rods for engine. I tried a few changes, and I've fiddled with numerous times, always end up putting back together the way it came.