As mentioned, COLD! is the word. Always change plugs in an aluminum head with the engine cold.

It's possible that whoever installed the plugs overtorqued it and messed up the threads. Any idea how long the plugs were in there ? How were the threads on the other three? dirty ???

Can you move it at all ? even a fraction of a turn ? If so, back it out that fraction, hit it with PB Blaster (ALUMA-TAP is probably a better choice, but it's hard to find, around here at least... while mainly used when tapping aluminum, it is an EXCELLENT lubricant when removing buggered up threaded things) ... ok, let the PB work into the threads a bit, now screw the plug back in. Work it back and forth, in and out. Each time you back it out, you should be able to get it to come back just a little more. Hit it with a little more PB and continue the process. In and out, etc... Take your time, let the PB do it's job. Pause now and then to let the heat that will build up from the friction of the threads to dissipate. Eventually you will get it out, but yes, you do run the risk of the threads being messed up.

Yes, you _can_ strip it when loosening it, but usually only because it was overtightened to start with. Something like 15 ft/lb is a reasonable torque for a plug in an aluminum head.

Or, are you saying the plug just won't turn, period?
Try tightening it a smidgen first... this also works pretty well with stuck brake bleeders.

Good luck!