My Express 4K+ used to just stay on all the time ...
Now it keeps turning itself off ... I have to push reset or disconnect power cable and plug in again to have it come on...
Is the unit faulty or is there some setting that needs to be reset?
It probably has "power saving" enabled. Disable it under Settings->System->Power.
You don't have to reset it or unplug it - just press Home on your remote to wake it up.