It's no different than if they put all 4 gears on the shift lever
itself.  You could shift manually from 1, up to 2, up to 3, then push
the button for 4.

If you're in an area where the transmission wants to hunt between 3 and
4, you might want to keep it in 3 to prevent that back-and-forth hunting.

Or you might be going down an incline where it's better to keep the
tranny in 3 and use engine braking instead of using your brakes.

Those are two instances where many people find themselves where they
want to control what gear they're in.