The Q5 version with the current body and clip must have come out sometime at the very end of 2008. It was followed by the R4 sometime at the end of 2009 and the eventually by the R5 around December 2010 - dates are approximate. So the R5 is the most recent version.
You may see different data (output, runtimes, throw etc.) by Fenix between R4 and the R5 but keep in mind that at some point Fenix changed the way they performed their tests to comply with the ANSI/NEMA FL1 standard so it may appear that the most recent emitter performs less well than the older one but is probably not the case unless they changed the way the circuitry works.
As a general rule, the newer emitter will be brighter by 7% and also offer slightly better runtimes. For example, the R5 can be expected to be roughly 14% brighter than the Q5. Such difference is not clearly visible or not visible at all, to the human eye. Runtimes would be detectable if timed. That said, I'd go - and did go - for the last emitter - i.e.: the R5! Much more so considering the price will be probably the same as the R4 or the R5 might even be cheaper.
If instead you find the R4 at a much lower price, then you might want to go for it as the difference with the R5 is not that big at all. I don't think there's a full review (including tests on OTF lumens and runtimes) comparing the R5, R4, and Q5 but if you search the different threads you'll probably find lots of useful information.
Great light and I highly recommend it. Even offered it twice as a present to my father: unfortunately neither flashlight ever made it through the mail system to its final destination which really pixxes me off! I'm still hoping the second one is just delayed! As for the first, definitely stolen but I do hope whoever got it gets electrocuted with it!
