The Mighty Bright is an excellent lamp, but it's way too big to use on a bare K4. I have a Touch, with a slightly larger bezel than the K4, and it's unwieldy on that. Great choice if you're using a cover though.
Another Solis user here, and it's the best book light I've ever seen. I've tried and used several, going back to my paper days, and this is the only one I've ever really loved or thought was worth the money. Used with a rechargeable AA battery, I've gone about 35 hours so far on a single charge. And I always have charged AA's around, so unlike the Verso, when the battery dies, I don't have to quit reading! Just plug in a fresh one and keep going.
I suspect the Amazon lighted cover is the best overall option, but for those of use who don't use covers, the Octovo Solis is an ideal choice. Unfortunately, it's currently out of stock at Amazon unless you buy it bundled with a Kindle Keyboard cover (and the cover doesn't fit the K4). It is still available on
Octovo's site (though I don't know if it would arrive before Christmas) and recent communication with them indicated they were doing a small tweak to accommodate the new Touch, so it hasn't been discontinued entirely. If you compare the reviews on it to those of every other light, I think you'll see it's worth waiting for:
http://www.amazon.com/gp/product/B004CX34KE/ref=oh_o01_s00_i00_details