black screen on remote computer when 4K monitor is off but shows up fine if on

A friend recently got a new 4K monitor and I can connect to his computer just fine to do what I need to from my house like I did before, unless the monitor is off.  When he actually turns it off with the power button at night (instead of Windows 10 power off after x minutes) I only get a black screen.  No desktop icons, system try, start button, ect.  I can change the view size, but it's all just black and nothing to click on even if I click where I think there should be something.  He turns the monitor on and it works just fine.  Also, while I am connected to the black screen, file transfer works just fine.  I'm guessing that's because it uses teamviewer's own file transfer interface for that.  I've never used the "show black screen" on the remote device because there was no one to hide the screen from. 

The computer I most often connect to the monitor is almost always turned off but it's not 4K.  So, is there just something about 4K that the monitor needs to be left on for remote access or might there be some setting I can change to let me remote in like normal?