2 inverted, 2 regular subs, same enclosure

I have a different opinion on this.  All four subs can share the same enclosure.  You will be adding volume displaced by only two subs, though, when you're calculating box volume.  And the cone areas of the inverted subs will indeed add to the overall volume, so bear that in mind.  Of course you'll want to figure in some internal bracing on this enclosure, and that displacement would have to be added up as well.

Inverted subs have to be wired on the opposite polarity than the normally mounted subs.  Here's a way to make that easier:  temporarily stick a piece of masking tape on the terminals of the inverted subs, marking the tape with the polarity symbol that is opposite of where it's put.  Then just wire the four subs according to the diagram that you are going to use, using the polarity on the tape as the rule.  The tape can be pulled off once everything is set up and tested.

doesn't it also matter on how he wires the subs to the amp? For example: if he wires them all in parallel and one is blown, then he would need to replace/repair the sub before playing the music again to prevent damage to the amp and/or the other subs. so if what I'm thinking is correct, then in certain circumstances you would have to wait till the system gets repaired before playing it at all regardless of it being a single dual, or quad chamber enclosure. Please correct me if I'm wrong, because I'm not a professional at this kind of stuff, I just thought this might be relevant

I'm not one to worry about a sub failing and causing a problem in a sealed box situation.  Look at it this way, and I'll use nice round numbers to make this simple:  you have 4 subs in a sealed 4 cu ft box, each sub getting 1 cu ft of air to work with.  For some reason, one sub fails, the wires come off, something happens to it and it quits working.  Now there are three subs using the 4 cu ft, which gives each one 1 1/3 cu ft.  Would this ruin the other subs?  No way.  They'll be fine until you fix the broken sub.  The sound would suffer in the meantime.

Just look at the amount of play in cubic feet you have in building sealed boxes for any given subwoofer.  But if this case were a vented enclosure, it would be another story altogether.

Rob drove home the importance of building rigidity into a 4-sub enclosure, and using separate complete baffles assures the strongest box, as he suggested.  With sealed boxes there is a lot of forgiveness in air space volume...but with vented boxes there is no leeway.  This case is a sealed box, and it is entirely up to the designer whether the subs share airspace or work within their own.

crazycreations, you're talking about there being a difference in impedance load on the amp if the wires slip off one of the subs.  That is true, but the load won't become lower in a paralleled wiring scheme.  If one sub out of the group is removed, the impedance will go up...not down.  (Play around with the parallel and series calculators found in the left column to see what I mean.)

An amp will go into protection mode if the load gets too low at high output levels...but you would most likely hear the difference caused by a sub failure immediately.  And in all cases where one of your drivers fails, an immediate fix is necessary.