I am installing 4 inch speakers in kick panels of my daily driver. It would be easiest to mount them if I can set them where the magnet is tight against the back wall of the enclosure I am building in the kick panel. Is it okay to mount a speaker with the full bottom of the magnet up against the enclosure wall? The wall will be plexiglass, not metal. Or do I need some kind of airspace or insulation between magnet and wall of speaker enclosure.

Plexiglass is fine with speakers. You can also purchase kick panels with built in speakers and all the wiring,etc. I have a pair and they really work well-plus looking graemlins/waving.gif good.

The only thing I would worry about would be wether or not it would cause a rattle when the speaker is vibrating against it.

I have revised my plan and will not make the magnet tight to the back of enclosure. I will have a thin space. Should I seal the enclosure I make with caulking compound on all edges, or is it fine to let sound waves leak out of the enclosure, or maybe should I drill holes in the front of the enclosure (beside the speaker) to let sound waves out?

Instead of making your own or caulking around the speaker, www.crutchfield.com (http://www.crutchfield.com) has styrofoam speaker houseings that protect the speaker and redirect the music forward. I don't remember the part number but if you search there, I'm sure you'll find it. I did the same thing you are and used these along with bass blockers and they sound and look good with no rattles or vibration. The only part I had a problem with was I had to cut some metal because I installed 5 inch speakers. I might have some pictures if you care
Mike

I'd seal the enclosure. It will only result in better mid-bass and mid-range response. Drilling holes ("to let the sound waves out") or "porting" is an exact science, and won't be effective with a 4" driver.
I'd also put a few strips of sticky-back foam weatherstrip on the magnet. With the tight clearances you describe the foam (compressed) against the rear metal will dampen any harmonics/buzzes that may be created.
I would also use some type of high-pass crossover (bass-blockers as mentioned). A 4" driver isn't close to being large enough to effectively produce anything below 400 Hz, so do it a favor and eliminate those freq's from it's operating range. Use NON polarized capacitors of at least 16V rating.

Simple Crossovers (http://home.earthlink.net/~j.delke/id14.html)

thanks for all the info. I will pursue your tips.