radio buttons: I want only one to be selected

So, what you're saying, I think, is:

you have a question with 4 radio button choices, and only one of them (one of many correct) is correct.

Which means you have the "correct answers requires more than one choice" option UNCHECKED in the question settings.

And, when you click a radio button, then click another one, the first one stays selected?

You can reset the question variable by putting an action at the root of the question page that uses the "Modify Variable" action and then setting the Value to "null" (i.e. leave the value field empty) and set the Modifation Type: Set Variable Contents. This will blank out the previous selection.

Or am I totally off the mark??

Dwayne