At present, I am building and testing for PC.
I plan on eventually testing and deploying to PS4 and Xbox one also…
Can I use 2k texture resolutions for these devices? Or should I be aiming lower?
Anyone know the answer? 
Thing is, there isn’t a definitive answer. It depends on so many things such as the size of your scene, how good your LOD’s are and many others. Simply saying ‘are 2k textures too much?’ is a bit too vague!
Having said that, I see no issues with 2k textures but you’ll really need to test it all for yourself. As I say, it depends on so many things that we simply can’t give you an effective answer here without more information.
It depends is the best answer 
Windows->Statistics->Switch to textures sets
Now you can see exactly how much memory you’ve used in current scene. So it depends on memory limit you’re aim to use. You can have hundreds of 256x256 textures or dozens 2k and so on. You have decide for yourself what amount of memory is acceptable for you and then balance textures out.
Ah, I see, I should have realized this - Sorry
Regardless of what devices they are you should start high and optimize down when the time comes. Optimizing for consoles can be a very time intensive process that depends a lot on the type of game too. Where you want to spend your visual budget, etc.