[Formerly Steve Wilkinson]

If there is any advantage to multiple-electrode plugs, it's simply that they might last longer. When you first install the plug with "equal" gaps between all four electrodes and the center post, the current seeks the easiest path, to the electrode that is microscopically closer to the post or otherwise presents the least resistance. When it burns the gap between that electrode and post slightly wider, it seeks the "new" path of least resistance, etc. etc. etc. So it's theoretically like having a plug that lasts four times as long because it goes through four life cycles compared to a single-electrode plug.

And the previous poster is quite correct: multiple-electrode plugs have nothing in common with two plugs each with single electrodes. In the former, you get one spark per Otto cycle. In the latter, you get two.

Stephan