Doing the standard configuration, yes, 4″ would be the approx. overhang.
If modifying using “duckbill” or as we call it, Chicago Style Arm Rial Molding, you can approach it one of two ways…
Either add a second step back layer to the top to match the profile of the duckbill rail you make or buy, or add a lower lip to create the step profile you need for your rail. When adding a second top layer, you will increase the height of the bar by 3/4″ (.75″) – doing the lower lip method keeps the same bar height. The lower lip is typically glued and clamped in place. You can add a few screws 1 3/8″ screws in place of clamps. Clamping is best.
The lower lip method will also extend your overhang, so if you want to do that mod, you can trim back the bar top as desired.
The only rule of thumb for over hang is that is approximately “one hand” deep. This was determined by visiting countless bars and measuring the overhang with my hand. It’s rarely deeper than that, but when it comes to bars, anything goes.