The S4 is compatible with the Qi (pronounced "chee" - it means, essentially, "energy" in Chinese) standard for wireless charging, but only if you insert a receiver coil. Those are available for $15-$25, but I didn't have one handy, so I tested with the new Google Nexus 7 tablet, which already has a receiving coil built in.
Both do the job equally well, but each has its advantages. The GMYLE is a flat pad, which is great for a bedside table. It's also much more affordable than the Tylt - as of this writing, about $26 vs. $100. To charge, you simply lay the tablet on the pad, centered so that the receiving coil inside the tablet is over the pad.
In contrast, the Tylt, despite also describing itself as a pad, is actually a stand. That's incredibly convenient for use on your desk - you can keep track of your favorite app while you work, or watch a movie while you pretend to work. Either way, your tablet charges while staying tilted at just the correct angle.
Another nice feature on the Tylt is a visible green light that indicates that the tablet is charging - quite reassuring, since the tablet won't charge unless properly centered on the stand. Also, on the Tylt, the tablet will only charge in landscape orientation, because of the placement of the Qi receiving coil in the tablet. In portrait orientation, the receiving coil is too high, and thus doesn't align with the transmitting coil in the stand. This would not be the case with a cellphone, I believe.
The GMYLE unit also has a light that indicates the tablet is charging. Solid red indicates that the pad is powered on and solid green that it's charging. It flashed red and green for me, whatever that means, but the tablet did seem to be charging. However, in any case, the size of the tablet makes it hard to see the flashing LED. You sort of have to peer under the tablet to see it. Here again, this wouldn't be a problem with a cellphone.
With both devices, charging is slower than plugging in the tablet. That's because the wireless charging devices output only 1 amp, rather than the 1.35 amps that the plugin charger provided with the Nexus 7 outputs. Also, there's some efficiency loss in charging wirelessly. So, I'd expect that it could take almost twice as long to charge wirelessly as by wire, but I didn't time it. The Galaxy S4, like most cellphones, comes with a 1 amp charger, so the speed difference would not be as great. In any case, the idea is that you're more likely to charge your device if you can just lay it down to do so.
That's about it. Check out the GMYLE wireless charging pad and the Tylt Vu Wireless Charging Pad on Amazon. And look for my next installment, where I examine USB chargers (don't yawn, I found some unusual stuff). Afterward will come USB cables (ditto, some cool stuff), video connections for the S4, such as connecting your cellphone to a TV, then Bluetooth keyboards and mice, how to use your cellphone as a portable office, and finally miscellaneous accessories. You can find all this at http://www.huffingtonpost.com/jonathan-handel/.
Disclosure: Manufacturers provided product for this review.