I tested right now on the 4k Monitor on my study.
I get a full screen Roon image and the display information says it is 3840x2160
So no black stripes and no further issues. As you saw in my videos it also works on my small HDMI monitor like charm.
As far as I know the monitor tells the Raspberry via HDMI its resolution and the Raspberry adapts to this. For me it works flawless. This means I have no idea what I can do to improve things for you 
Was your monitor connected before booting the Raspberry?
Maybe your monitor is too old and does not support this feature?
Best DrCWO