If you compare part numbers for parts in the carb at Marineengine.com or similar place, you will fined that the nozzle Part #28 (the venturi) and the throttle valve part #3 (butterfly) are different part numbers. I think the rest is all the same. Since the high speed jet is adjustable, the 4 HP carb may work OK on your 3 HP – I’d try it! Not sure, but my guess is the small holes in the throttle valve are probably a different size. I ran across similar differences a few months ago between a 7.5 and a 9.8 Mercury carb. The holes in the throttle plate control how much air it gets at minimum idle where the throttle is closed all the way, so on your 3 HP, the 4 HP carb may not idle down quite as slow as the 3 HP carb – but if that’s a problem, plugging one hole may help. Let us know what you find – you’ve got me curious now.
Looking again – part #23, the high speed needle valve is also different part number, but that could just be a different length or knob attachment detail.
Dave
Trying to post a pic below:
http://www.marineengine.com/parts/evinrude-johnson-parts-diagrams/EVINRUDE/39864.gif
