I think #4 being Manifold vacuum, but I'm dumb a lot. At any rate its the one I use for Vacuum Advance.
There are 2 types of vacuum available at the carburetor. Ported Vacuum and Manifold Vacuum. Ported vacuum originates from a source ABOVE the throttle blades. There is no vacuum at closed throttle, but vacuum increases as you open the throttle. Manifold vacuum originates from a source BELOW the throttle blades. Manifold vacuum is always present with the engine running, at closed and part throttle. With the engine running, it EASY to tell the difference. Cap all the outlets and start the engine. Remove one cap at a time. If it hisses, that is Manifold vacuum, if it doesn't, it is Ported vacuum. Put your finger on the outlet and open the throttle, you'll feel vacuum. Both Manifold and Ported vacuum go to zero at Wide Open Throttle. You can use either for vacuum advance, and opinions abound. Using manifold vacuum will advance timing at idle and part throttle. That can be useful in some combinations. The 1 and 2 in your first picture are unclear as to where they go. That vacuum break in the picture needs to have manifold vacuum. In the second picture 3 is usually manifold vacuum, 4 is ported vacuum. Test them to confirm.
The regular connection to the timing advance is the ported vacuum port (number 4 in the photos), so that a small amount of advance is present at part-throttle highway cruising speeds. It's supposed to improve fuel mileage and let the engine run cooler. The port is inside the drivers-side primary bore, and is covered up by the throttle blade edge at idle. Most performance setups have the advance on the manifold vacuum port, but for normal street use: use the ported one. Number 2 should go to the PCV valve.
My advice is to try both ported and manifold. See which one you like better. At part throttle cruise, vacuum advance will be the same. It's not like it is hard to switch.
