Perhaps you have already heard it through the grapevine, but just to be sure, I’d like to mention that VileR (of 8088 MPH fame)has patched the Keen 4 code and redid all the graphics to make the most of the 16-colour composite CGA mode.
It started with a tweet from John Carmack, after he had seen the video from the 8-bit guy covering CGA graphics (and featuring 8088 MPH):
I never knew about composite CGA optimizations!
So the original Keens had graphics designed for 4-colour RGBI mode only. Well, challenge accepted!
I will leave you with some captures from my own PCs.
First my IBM 5160 with a new-style CGA card:
Then my Commodore PC10-III (8088 at 9.54 MHz) with its onboard Paradise PVC4:
And finally, the PC10-III with my ATi Small Wonder card:
The graphics were originally done for old-style CGA. As you can see, the new-style CGA is more saturated, but still acceptable.
The Paradise PVC4 is very saturated as well, and the colours are slightly off, but still somewhat recognizable.
The ATi Small Wonder is completely off however.