What Gordon said, with the clarification that Snap is really a shortcut that Ricoh provides to Zone focus. It's just a different and quicker way to set your zone focus distance, but to use it effectively you still need to understand DOF and set your aperture accordingly. And some Ricoh models have an infinity setting among their snap focus distances, so it's kind of a shortcut for that too. And you can get pretty close to hyperfocal shooting with snap too, but you can't fine tune the distance, so you can approximate it or slightly overshoot it, but rarely hit it exactly - but a slight overshoot isn't gonna hurt you if you're trying for hyperfocal.
One warning - don't trust the DOF scale on the GRD4. The one on the GR and GXR28 are really good, but the ones on the smaller sensor GRD models are waaaaay off, so spend a little time with a DOF calculator to fully get to know how to best use snap focus on the GRD4. The distance scale is fine, but not the DOF information.
-Ray