Found a solution: https://blender.stackexchange.com/questions/264754/how-to-deselect-one-keyframe-every-nth-on-a-multiple-objects-selection
You have to enable Developer extra and then use F3 to search for Deselect. It has every 2, every 3, and every 4. I’ve tested on 4.2 and 4.4 and seems to work fine. If I find the time I’ll see if I can link the operations to a menu.
