Another thought....try using a mouse with a pointer with higher resolution or use a higher dpi value for what you have?
I'm gonna have to figure out how that works. I never knew you could change mouse pointer resolution ...
But at at least by adding the 4 last lines from Steven's config.ini, I managed to get Il-2 to work on my 4k screen, BUT in a 1920x1200 resolution. Which means there is a large black border all around the game, but it does work as it should. So that's a partial victory ... for which many thanks.
I can't say what resolution the game is actually on screen. Cause at 1920 that would mean 1/2 the size of a 4k screen, but it looks more like it takes in about 3/4 of the screen width. I've no idea how to measure pixels on a screen.
And in any case, even in 1920x1200 mode on a 4k screen, the color and quality is way above that of an old, non-IPS screen ...