I have the
Lighted cover for the K4 SO. It has a tiny light that swings out from the top center and it runs on the K4's battery power. Small thing, but it covers the whole screen pretty well, better I think than the 45 degree angle pull-out one on my K3 WiFi/3G. Overall I like it.
Like TubeMonkey, the K4 SO is my preferred e-reader over the K3. Because it seems so much lighter and smaller than the K3. Actually it is only a few ounces difference, but it does make a difference if you read for long periods.
I can't say about tethering it to your phone there, but I have a mobile hotspot on my Droid X phone and the K4 catches it just fine.