Yes, that's exactly right.  The #'s 1 and 3 are for the same thing; the crankcase pulse to the fuel pump in the carburetor.  Usually, when a carb comes with that number 1 fitting, the number 3 is a "blind hole".  It doesn't actually go through to the pump and the number one is used as you said to connect to a fitting on the crankcase.</p>

Number two goes to the fuel tank, and number 4 gets blocked off as it won't be used.  It was for a primer bulb in the original application.</p>

The fuel tank gets vented to the atmosphere; not to muffler pressure.  All hose fittings should be secured with twisted wire clamps or small zip ties, as the fuel hose will sometimes swell a bit when exposed to gasoline.  If there are any air leaks in the fuel system, the fuel pump in the carb won't work.</p>

AV8TOR</p>