I got an LG 49UB8500 today to use as a monitor. My computer is a Toshiba S75 with an Intel HD 4600 graphics card on the motherboard. The graphics card supports UHD 3840 x 2160.

All programs display correctly at the UHD resolution, but X3 seems to go no higher than 1080p.

Has anyone else had this problem?

Turns out that my new tv/monitor was set incorrectly in "Change the size of all items".  The tv was displaying all applications too big.  This is using the NVidia GTX 750OC on my HP M9040 DAW that I installed this morning.  I am getting a full 3840x2160 display on an LG 49UB8500 tv and 1920x1080 on a 27 inch monitor.  Tomorrow I am adding a 37 inch Vizio tv as a 3rd monitor.
 
The GTX 750OC is cool because it is only a 6" card and draws only 50 watts.  It does cover two  PCIE slots, but I am running the Gigabyte card with 2 HDMI and 2 DVI outputs.  It outputs UHD at 60hz even thought the box says that should require 2 hdmi cables.
 
It is interesting because a display this size, along with the 37" Vizio and the 27" Acer pretty much is the limit of what my eyes and head can handle.
 
 

Let's see it.

https://www.dropbox.com/s/orfif4xv30kldy9/Untitled.jpg?dl=0
 
The jpeg was too big to paste here , so I put it in dropbox. 
 
I can get about 35 tracks from console view on the screen at once, but I normally don't use nearly that for what I do in X3.  I also found that the monitor is so big vertically (nearly 24 inches) that it is best to put what you use the most on the bottom so you don't have to look up so much.  The display on the left is my 27 inch  acer monitor for comparison.  It is little more than 12 inches vertically, Also I need to move my studio monitors to accommodate the new configuration.

Bluegrasser
I also found that the monitor is so big vertically (nearly 24 inches) that it is best to put what you use the most on the bottom so you don't have to look up so much.

I ended up sinking my monitor 8" into my desk after cutting 3" off the desk's legs so I wouldn't have to look up, which I found very hard on the neck.  It's a 30" monitor and the screen is 18" high and on a stand which raises it a couple of inches.  Now I look slightly downward with the screen tilted to be normal to my line of sight and it's very comfortable.
 
Your screen is quite a bit bigger and I'd definitely look for a way to lower it so you're looking down rather than up.
 

One thing I will say about the UHD tv is that you can still see clearly while sitting pretty close.  I am using a dinette table for my computer desk and I'm writing this with my eyes about two feet from the monitor.  The pixel density is so great that I can't see the individual pixels.  It is every bit as clear as the 27" Acer, just bigger.  When I add my 37" Visio at 1080P I will see the effect there of lower pixel density.

Bluegrasser, are you finding the text larger or smaller on UHD TV versus your 27" LCD? 
 
Cheers,
 
jayson

Thanks for the pic!

I am using Samsung 28" UHD monitor. I will never go down on resolution now. My setting is 150% bigger in screen... what's the word? Nor resolution but making things look bigger without destroying resolution? It works. And I love it.

For Jayson:  My text is the same size on both monitors.
 
I have found myself moving things to the bottom center of the 49" monitor as I need them.  For example, if I am working in track view I just grab it and move it to where it is comfortable.  If I need lyrics and console view I can put them side by side with lots of room to spare.  I mostly use the 27" monitor to show synths, file browser and other desktop stuff.  If I need that stuff I can grab it and put it on the 49" monitor for a second until I'm done with it, and then I put it back on the 27".  Also, I have made the console and track views smaller than what you see in the jpeg I posted because I normally only use less than 15 tracks and busses.  I can keep my X3 user's manual open on the 27" for reference
 
One downside is that I have to figure out a new place for my studio monitors because I now have a 37" Visio tv as a third monitor.  Most of the time I just use the 37" as a tv on the side, muted of course.  Also, I'm not happy with my boom placement for my microphone.  It has become a question of where do you put hardware when you have these big monitors.
 
I'm very happy with my LG 49UB8500 tv and the Gigabyte GTX750 video card.  If you are thinking of going this route, be careful about which tv and video card you get, and make sure all the inputs on the tv and outputs on the video card match your requirements. Also watch your computer power requirements.  Some video cards need more than the 500 watts I have in my machine.   A lot of info on the internet is out of date.  My GTX750 box said it required 2 hdmi cables to do 60 hz (I am not a gamer), but when I plugged a single cable into the hdmi 3 port on the tv, the tv saw 60 hz.  I might try using the other hdmi ports on the tv to see if they don't support 60hz also.