Strangely enough, three out of four of them are people whose Wikipedia pages I've consulted in the last 24 hours. Probably not the answer you're looking for, but now I've looked at Mark Williams' too. Because of that, I know that three out of four were professors in the TV adaptation of Gormenghast.
None of that sounds like a factor for all four actors, though …
Nope, just weird coincidences! One of the wiki visits was your fault, I just realized—Carlyle was in the World is Not Enough, which I was considering revisiting after the last round of FAF.
Now I want you to actually find the thing they’ve got in common, though
The best I can offer so far is British television small-town dudes (Hamish MacBeth, Doc Martin, Peter Kingdom, Father Brown), but that's probably not specific enough.
That’s exactly what I was going for.