Need TVs which behave more like a monitor (as below).
Only 4K mainstream priced TVs, 40-60 inches, supporting RGB 4K60 over HDMI 2.0 ports are considered.
40 inch+ monitors are very expensive, 40 inch+ TVs are very cheap.
(1) Monitors will turns on/off automatically (unless it is "full off") when the input signal is on/off, but most TV doesn't have this feature, they will just display a no signal message when the input HDMI signal is off.
(2) Monitors will switch to another HDMI signal that is on automatically when the current displayed HDMI signal is off
(3) Better to be non-android TV which is to slow to turn on, but since most 4K TVs are android based, the more realizable goal is the TV have a standby mode and the time from active to standby and from standby to active is as fast as possibile (for comparison, for phone and tablet this is less than 0.5 seconds, the fastest TV I'v used need 6~10 seconds)
(4) Monitor will display the image when on, but for most TVs the default scene is the home page and you need operate the remote to display some HDMI signal. Besides for TVs switching between HDMI signals need multiple remote button clicks. Monitors can also remember the HDMI channel displayed last time and you don't need to swtich channel each time.