You could try 720p mode as you have a Pi 4:
Full config details are here on the RPF site.
Add the following lines into /boot/config.txt
and restart the computer.
To use dmt add:
and restart the computer
Try the following in /boot/config.txt:
# force HDMI
# disable EDID
# DMT group
# 1366x768 60Hz 16:9
This in fact is a known problem with the Sense-Hat. See https://github.com/astro-pi/python-sense-hat/issues/96 for detail.
Apparently, the Sense-Hat interferes with the resolution auto-detection if there is no monitor attached (works fine with monitor attached).
The work around is to manually specify a resolution (you do not have to force HDMI). You can ...