I would recommend waiting to see what monitors come out leading up to the November release of the XB1X. The 1X supports HDR, so you really should consider getting a monitor that has HDR support (HDR10 specifically). Currently there are not a lot of choices on desktop PC monitors with HDR support (especially at 4K). But that's going to change as the panel makers increase their HDR HDTV production. The question there of course is if any of them will have Freesync support (something I'm not sure is necessary with a console running a 4K monitor since games are going to be 30 or 60 FPS capped anyway).
FYI real HDR approved monitors are at a minimum of 10-bit panels and have 1000 cd/m2 nits of brightness. Most high end monitors are 200-300 cd/m2, even 10-bit panels. There are more specs that must meet HDR standards, but those two are the most important in taking full advantage of HDR.
TL, DR: no point in buying a monitor before you have the XB1X in hand; the longer you wait to buy a monitor, the better your options are (and you might even pay less three months from now).