L.E.D.'s are binned according to, among other things, brightness. A "Q" bin is the natural evolution from a "P" bin. Check the Cree chart below:



As you can see a Q5 is about 26 lumens brighter than a P4. But from what I have read the difference to the naked eye is not as much as the numbers may indicate.

The first R2 binned lights came out a few months ago. If you're not in a hurry just wait and I'm sure Zebralight will follow suit.