I have used this Latitude 7450 for about 8 months with a Seiki 39 inch 4k set (one of the first cheap HDMI, so was only 30 hz capable, but it was good for the kind of work I do).
I recently upgraded my home PC display to a Samsung KS 8000 quantum dot 4K, as my Seiki stopped turning on. For the first few days this worked great with the laptop and desktop, I just had to use remote to switch between them, but last night the laptop stopped seeing the KS 8000 as 4K capable and would not go beyond 1920x1080.
Some further history ...
In the past, randomly the laptop display would reset resolution to something like 1280x720, and most of the time I just had to manually set it back to 4k. It mainly did this when exiting webex sessions or when the display would go off/etc. It hadn't done this particular glitch with the new display, so I thought maybe somehow the Seiki was the cause and was hopeful this would not happen again.
Unfortunately ... I'm now here because not only did it flip itself down to the lower res, it will no longer go back up to the proper resolution.
What I've tried
Confirmed all cables I'm trying on laptop and all ports on HDTV have worked with my desktop PC (GTX 1080) which drives the 4K just fine on the TV without glitches of any kind.
Tried removing HKCU/CurrentControlSet/GraphicsDrivers/ entries for the monitor to "reset" it (I had to do this in the past with the Seiki on occasion but it didn't work here).
Multiple reboots with/without the separate display plugged in, starting up with it as a secondary "extend" monitor, as duplicated, by itself, etc.
Tried upgrading graphics driver to the latest.
Tried using mini displayport to HDMI adapter (which I confirmed works fine on my Surface Pro 2)
Unfortunately I'm stuck with this laptop as it's for work, so I can't change it, or I'd gladly replace it.
Have you updated the KS800 Firmware from the Samsung website? If not, try updating it to see if that may resolve the issue. Post back with information. Click the link below to be routed to the Samsung Website. Also, verify that the link goes to the correct drivers page for the 8000.
According to the TV autoupdater, I had the latest firmware, but apparently there was a newer one available through the website that I could install only through USB. I installed it, but this didn't seem to change anything, the laptop still only thinks the display has 1080p capability.
Here's what's so weird about this. The Latitude 7450 has an intel HD 5500, which should be better than the HD 4400 in my Surface Pro 2 tablet, and yet I can use that using the same HDMI cable (I have a mini display port to HDMI adapter for it) and it broadcasts full 4K resolution with no problems at all.
I've tried adding a custom resolution using Intel's graphics properties option off the right click menu, and it tells me "Exceeds Maximum Bandwidth" when I try to add 3840x2160 @ 30 hz (what I was able to get before it glitched out).
I've tried using the mini display port to HDMI adapter from the surface pro 2 with Latitude as well, and it still only gives me the 1080p option.
I found some information that may be helpful resolving the issue.
HDMI Micro Connector - A new, smaller connector for phones and other portable devices, supporting video resolutions up to 1080p.
What kind of cable will I need to use for a 4K display?
A High Speed HDMI Cable (with or without Ethernet).
What cable do I need?
Use a standard cable if you have a lower-resolution display (approx. 720p or 1080i).
Use a High Speed cable if you have higher-resolution display (e.g. 1080p). High Speed cables can deliver all video content currently defined for the HDMI standard (i.e. more than two 1080p/60 video streams, including 3D).
Use an HDMI cable with Ethernet if your devices take advantage of the HDMI Ethernet Channel. HDMI cables with Ethernet are capable of sending 100/mbps over the HDMI Ethernet Channel.
Thanks, unfortunately I know I have the high speed cable, the one I am using is below (which as I mentioned above, I've also tested and confirm it works with a Surface Pro 2 that has the Intel HD 4400, which in theory should be less powerful than the HD 5500)