Really interesting question, Derek! I cannot determine if you mean that all 4 have the same difference, or if it's that all four have a unique difference.

If it's the latter, then as for "Rest in Peace, Mrs. Columbo," is it that Columbo and his wife are the targets? I cannot think of another episode where that is the premise, though I can think of a few where the murderer (at the end) came very close to offing Columbo, such as the Shatner one where he pulls the rifle out of his trunk.

Edit: I believe that is also the only episode in which we see Columbo's home (unless you count his apartment as a home, because we see his apartment in "A Friend in Need").

Just to clarify, I'm arguing that the four episodes form a group which is a group which shares certain characteristics quite distinct from all the other episodes.

Your observations wendy were very astute.

A clue: in tillyards brilliant essay he says that Hamlet, for example, is unlike the other plays because the predominance of the drama seems to be in Hamlet's mind rather than on the stage. Do you get the idea...?

Gosh, that is a tough one. I watched them over the weekend, and while I'm sure there's something distinctively different, I am unable to pick it out. It will be interesting to see the reveal!

The only one I'm pretty sure about is DEAD WEIGHT. It has a partial witness to the murder.
(Of course, DAGGERS OF THE MIND has a witness to the accidental killing, but both characters are actually part of the killing, so that puts it in a different category.)