ORIGINAL: MinnFlyer
I don't know that you can use a colder plug on a 4-stroke. They need the extra heat to get past the second stroke
Heat range and the persistance of heat are two completely different things.
The persistance of heat is a function of thermal inertia and requires more mass in the glow plug element than we are accustomed to with standard glow plugs. One can have a glow plug that stays hot longer, but which may or may not be as hot during normal running.
Ed Cregger