everything you could want to know once you start going beyond a single subwoofer.
http://stevemeadedesigns.com/home/
Mounting 4 subs like that will NOT cause any sound to get "muddy" if your speakers are working correctly and the box is designed correctly. Theres a guy putting 8 12s in his extra can Ranger and they all share the same airspace. Building a bunch of single boxes together will not make anything easier cause now you are having to cut more, screw/nail more, and seal more. Generally, you'll want all your subs running off the same signal/wiring and Ive seen up to 24 speakers wire off the same single bridged channel coming off an amp. Car audio these days has gotten OUTRAGEOUS with 10000 watt amps and multiple alternators to provide power. 5 or 6 320 amp alternators is a LOT of current.
and that pic of the 4 JL Audio subs is most certainly its finished design with plexi. You will rarely see a box that is finished internally and be all closed up.