Can I stop at the 400 grit and polish granite like I do with marble?

Unfortunately granite doesn’t react like marble. It will be necessary to proceed all the way to the last grit before polishing.
Marble can be polished from the 400 grit due to the fact that it reacts with oxalic acid. Granite does not react with acid and therefore you cannot take this shortcut.

FYI There are several diamond companies developing shortcuts to granite polishing. Watch the trade journals and newsletters for further info.