Why do you want a colder plug? That is old school chevy tuning stuff. Put a set of ND Iridiums in and leave it. The VR4 runs the same plug as the NA engines.
Your combustion chamber is the same as the late 2.6 engine and using a Starion for example they do not need cold plugs.
I can not see that you should need a cold plug if Mitsu did not see the need to in the production engines.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.