It works with EGA and the VGA modes
It does, but I'm curious what registers are used since they're apparently supported by DOSBox, but not the CGA ones.
Most copies (if any) of KQ1SCI didn't even ship with a CGA driver and using Tandy only gives you the one voice PC speaker instead of the 3 voice +noise channel that the AGI games give you with Tandy in DOSBox.
Are you sure you're not basing that off of copies obtained from abandonware sites? Because my original KQ4 disks have the full complement of CGA and Tandy video/sound drivers.
But then it's possible you're right because most Sierra games from 89-92 shipped on 720k and 1.2MB floppies. They appear to have been designed for newer systems and may not have CGA/Tandy drivers. Whereas KQ4 (being an older game from 1988) shipped on 360k and 720k disks and was probably still made with the idea of 8086 support.
I can also recall at least one non-Sierra game (Defender of the Crown) that shakes the screen.
That said, it would be desirable to have it working with Tandy for the AGI games
AFAIK, the Tandy CRTC registers are accessed and set up the same way as CGA (although some of the values might be different in Modes 8, 9, and A). Therefore if we add code to DOSbox's Tandy mode to emulate the HPOS and VPOS registers, it could then also be put into the CGA mode with no modifications.
This could be done for the PCjr as well (remembering that AGI games have to work on there too) but the registers aren't accessed the same way as on Tandy/CGA.