Found this link really helpful:
These are the changes to config.txt that worked for me:
# Enable DRM VC4 V3D driver on top of the dispmanx display stack
dpi_timings=480 0 10 16 59 800 0 15 113 15 0 0 0 60 0 32000000 6
Disabling vc4-fkms-v3d will probably mean that the HDMI ports will no longer work – I’ve yet to check, but as I mentioned originally I’m just wanting the console on the Hyperpixel 4.
Rather nicely, ‘startx’ brings up X properly on the Hyperpixel 4, and touch is working just fine. (I did earlier run hyperpixel-rotate when I first started on this, so that may be a necessary step for this to work.)