Xûr is a universal vendor, he sells the exact same things each week across both consoles. Even goes to the same location. The reason he can't sell the PlayStation exclusive weapons is because, well, they're exclusive and Xûr is not. For Xûr to sell the Hawkmoon for PS4, it would also have to show up on Xbox, which it genuinely cannot until the timed exclusivity runs out. Xûr will be able to sell the Hawkmoon and so on when Xbox gets it around the launch of The Taken King. Same thing goes for Nightfalls, they're universal and that's why you don't see any PS exclusive strikes as the Nightfalls. They'll be able to be used for the Nightfalls once they're available on Xbox as well. That's just the way it works. Even if Xûr COULD sell the Hawkmoon, it doesn't mean he would anyways. Just keep trying to get the Hawkmoon to drop, everyone has that last exotic they can't seem to get for however long it may take. Mine was Hard Light. You'll get the Hawkmoon eventually. :)