I'm going to be checking the spark plug on my recently purchased 400ex soon (well, as soon as I buy the honda socket wrench) and if I need to re-jet it, what size jet should I start with? I'm most likely just going to buy individual main jets for the local dealer, instead of an entire jet kit.

I'm at 4,000 ft elevation in the warm desert. The quad has a Pro Circuit T4 slipon and a K&N Filter with a modified air box (holes drilled in lid).

I'm not sure if the previous owners had it jetted for there location or not, but the location where I purchased it was at 2,000 ft elevation so either way I'm sure it will need to be re-jetted again. I haven't been able to check the plug yet because I don't have a spark plug socket that will get the plug out, but the quad seems to run great even at this altitude.