You don't need to limit yourself with 4 mics. On most devices like your Korg, the TRS inputs are line level inputs though, not mic inputs. Meaning that there's no preamp in the circuit. To avoid stuffing my foot in my mouth (as I've done many times before) I checked the specs on the Korg website, and although the TRS inputs don't have phantom power, they have the same input impedance as the XLR inputs. Basically, this means that they do have preamps. If you want to use mics on those channels, you'll just need to get cables with XLR on the mic end and TRS on the Korg end and you're in business.
You are correct in assuming that most guitars do not have balanced outputs. Some electric/acoustic guitars do. Most of the time this will be obvious as there will be an XLR jack on the instrument. Some guitars also have stereo outputs. The jack looks and functions like a normal mono connection if you plug a regular guitar cable into it. If you use the appropriate cabling though, you can send different strings to different places in the mix. This scheme is further confusing because some guitars have 3 jacks. One for left, one for right, and one for the standard mono output. In all cases except when there is an XLR jack, the signals are unbalanced. Whether or not the instrument is using a TRS style cable or not has no effect on the 'Balancing'.
As for the POD, The PODxt has balanced outputs. They can also be used as unbalanced outputs simply by using a regular cable. Even with the balanced cables, they're a bit noisy for studio work (IMHO), and the connection from the guitar is still unbalanced. It's usually a good idea to keep the unbalanced cable as short as possible.
That said, I thought I would explain exactly what is meant by 'balanced' vs. 'unbalanced. Perhaps this is more information than is necessary, but I'm bored, the lawn is mowed, it's sunday, and the kids are finnally someplace else making the usual kid noises.
Really, it's fairly simple. In an unbalanced system, the center conductor carries the signal whilst the outer conductor (shield) carries the return or gound. Any noise that the cable picks up along the way should bounce off of the shield. Noise can be a nasty, persistant bastard though, and some of it will usually find a way through the shield. Now that there's noise on the inner conductor, it gets passed along to every other piece of gear connected to it. Usually ending up eminating from your wonderfully accurate near field monitors, which faithfully reproduce that pesky noise in the form of hum and buzz and the occasional pop when the fridge turns on.
In a balanced system, whether it be XLR or TRS connectors, there's 2 conductors surrounded by a common shield. The output of your balanced gear spits out 2 seperate signals, one for each inner conductor. These 2 signals are identical except in one aspect. One of them is 180 degrees out of phase. So the kids want something to drink and open the fridge again. As usual, they leave it open and the poor thing kicks on in a vain effort to keep my roast beef cool. On the other side of the house, my balanced cable just picked that up, just the way the unbalanced cable did. Same noise, but now it's on 2 conductors, not one. The next piece of balanced gear in the signal chain gets these 2 out-of-phase signals and compares them to each other. In a perfect world, they should completely cancel each other out (remember they are out of phase). Well, the fridge noise is the same on both of them (in phase) and hence doesn't cancel out. The said piece of gear sees the remaining signal (fridge noise) and throws it away. What's left is a noise free signal to do with what we please.
In effect, the balanced cable does nothing at all by itself. It's the gear at either end that is doing the work. It's the analog equivelent of digital error checking. The TRS connector in itself is used because it takes up less space than an XLR.
Something to note. By far the most common use for TRS is actually unbalanced. Insert jacks on most mixers are using one of the inner conductors for the outgoing signal and the other one for the returning signal. This reduces the required number of jacks by 50%. Since these to signals are not the same (you just passed it through a compressor or something) there can be no error checking and hence no noise reduction. If a peice of gear has a 'Send' AND a 'return' jack that are TRS, the signal is balanced and you get to reap the benefits of noise free recording, even if your roast beef is a little warm.
