The P4 means you are guaranteed to get a P4-binned (flux bin that is) Cree XR-E in your flashlight. So in your Fenix you might get a lower flux bin XR-E which is less bright. More information about binning : http://www.cree.com/products/pdf/XLamp7090XR-E_B&L.pdf
Thanks for the link!
A bit odd that Rexlight guarantees a P4-bin when their light is almost half the cost of the Fenix. Feels like Fenix has lost their grip a bit, remember when they guaranteed bin in L1p.