Emmett Clayton from "The Most Dangerous Match."
Adrian Carsini from "Any Old Port in a Storm."

And Tommy Brown (from "Swan Song"). Although we only see them just sitting in Columbo's car, we have to infer that they eventually did drive away together!

Weren't they in the brown rental car at the end of the show? I don't remember Cash in the Peugeot.

Yes, they drove away in Cash's car with Columbo at the wheel.

Let's not forget Anne Baxter in "Requiem for a Falling Star."

do we have to actually see them in the car?
if not i would say ruth lytton from old fashioned murder, because there were no uniformed police around, and columbo escorts her out the door...assumedly to drive her himself to the police station.

E and Sean are responsible for coming up with three of the four: Adrienne Carsini, Nora Chandler, and Emmett Clayton. Who is the other?

Roger Stanford rode with him, along with Logan, on their way to the tramway for the final scene.

Very good, Frank. You have completed it.