I don’t know if this is common knowledge or not but I recently acquired a hyperpixel 4.0 touch and very nice it is too.
Upon a clean install of Raspbian OS I installed the hyperpixel lovely. Them I thought I wonder what happens if I plug in the HDMI cable too a monitor too…firstly nothing but then on a reboot it worked and I had duel display hyperpixel and monitor. This also worked with the official raspberry pi display.
Secondly. I was under the impression that the picamera python library would not display live output on the hyperpixel or pi display. I tried and it worked on both devices.
Just wanted to post this as help incase anyone was searching for information.