The problem is that when attempting to use multiple sound cards, you will have multiple different clocks. Unless you sync up the sampling clocks properly, the tracks would be very difficult to sync up in your software. More sophisticated rigs solve this problem by having a separate clock source (house clock), and distributing it to all the hardware, so that everybody stays in step.
What you really want for this is to get an 8-channel A/D converter, and a sound card that can input 8 channels all at one whack. Trying to do it two at a time will probably leave you rather frustrated. But trying to do it with 4 different cards will leave you much more frustrated.
Here's the problem. Let's say that you are sampling at 44.1 kHz, and you have 4 sound cards all set up, each running all by itself. Each sound card will sample the audio according to its own clock. And regardless of anything else, those clocks will all be running at slightly different speeds- no oscillator (even crystal controlled oscillators like were talking here) is perfect.
So let's say that you have one card that has a clock that is exactly, dead-nuts-on, 44.1kHz. And then lets say that you have one that has a clock that runs 1% slow, and one that runs 1% fast, and one that runs 2% fast. You pull in all 8 tracks at once, and then what do you have?
You probably have one hell of a bunch of pops and clicks, because the audio software can only sync up with one of the four sources *anyway*. Or, just for the sake of discussion, let's say that the software somehow magically _can_ do this: so you have 2 tracks that are the length of the tune, and in tune; 2 tracks that are 1% too long, and 1% flat; 2 tracks that are 1% too short, and 1% sharp; and 2 tracks that are 2% too short, and 2% sharp. Everything starts out at the same time, but then the tracks drift apart, because of the sampling rate differences.
The precision of the sampling process is _everything_: sampling at one rate, and then playing back at a slightly _different_ rate, results in timing distortions and frequency shifts. So to use multiple sampling devices, you *have* to have a single clock- or timing, tuning, and general functional problems will be the name of the game. It's not pretty. You don't want to go there.
If you have to, fly them in two at a time- but then you pretty much have to slide them by hand to line them up (unless you can somehow set up timecode, which I doubt). By far the best solution is to fly them in all 8 at once in parallel...
