PINN & NOOBS only work on the primary display (HDMI0), the left one nearest the USB-C power socket on a Pi4.
The other indication that you are using the incorrect monitor is that the secondary display (normally HDMI1, the right most connector away from the USB-C power socket) will display the rainbow screen, but the top left corner will be magenta rather than red.
The boot sticky actually says:
Which means that if you have a 4Kp60 monitor, you should plug it into HDMI0 because that's the only port that outputs that specification. But both ports will work adequately with a 1080p60 monitor.If you want to use a 4kp60 monitor you must use HDMI0, so If you don't get a picture out of your 4Kp60 HDMI monitor you might want to try it with the primary HDMI port, which is the one located near the USB-C power port. The other (right) HDMI port will not generate a picture beyond the rainbow GPU test screen, when you use a 4kp60 monitor.