SCOTT42,

                     Although some folks use the OS #7 glow plug in their four strokes OS states that it is primarily a two stroke plug.

I would recommend you use the YS four stroke plug, also the OS type F or any of the Merlin four stroke plugs.
(Merlin recommends the #2009 plug for inverted applications)

http://www.merlinglowplugs.com/Aircraft.html

YS plugs at...

http://www.centralhobbies.com/

Happy flying, Oscar