I put a hotter plug in my 400ex cause it wasn't running to well in the cold. It seemed to help out at first. When I finally get it started not it dies when I give it gas. I was thinking about getting a space heater and puting it in my shed with the quad and heat it up before I try to start it. Would this work? If not, what will?

The space heater will work initially but what will you do if you park it outside for a few hours?

You might try fine tuning the choke, turning up the idle and enrichening the pilot jet screw.

A space heater would be a great idea. The 400EX is cold blooded and there is no special way of starting it in the cold, people have there tricks on starting it but it is all the same to me. My 400 starts better now because I have an aftermarket carb, but some kind of heater by the quad would work the best!

It sounds like you need to adjust/replace the pilot jet for the cold weather. Also, the 400ex I had started all year around good, but if I appliad the throtte a couple of times, it would not start for a while.

You may want to be careful with the hotter plug. The combustion chamber heats up hotter in the winter because the dense/cold air leans out the mixture. A hotter plug will create even more heat and could burn the piston or ruin your valve seats.
The space heater sounds safer. Maybe turn out the air fuel screw a half turn

This is going to sound really weird, but whenever my 400 doesn't start in the Wisconsin cold, I loosen the carb bowl drain screw and let some gas flow through. After that, it seems to start with full choke. Maybe it's just a coincidence, but it works for me!

Well it was -30 with windchill and me and a buddy went ridding. To get it started i always give it 3 flips of the throtle before even pressing the starter.
One question. I get it started and i nurse it to idle, after a half hour it won't idle no matter where the choke is. But if i ride it really argressive it will idle after another half hour. Any suggestions? I was thinking of getting a filter and jet from K&N???