Copy and paste your relevant code here. Would help.
For instance, Flex SDK 3.5 test with 4 radio buttons and it works as it should:
View source is enabled.
Maybe you have some other element, invisible, laying "on top" of that last radio button. I tried to break it by overlaying a canvas and image component but the button was still clickable. Doesn't mean that can't happen though. Maybe move your radiogroup somewhere else just to see if it still occurs.
That was it. A neighboring text field was wider than it needed to be
and was overlaying the affected button. All is well now.
Glad I could help. I've run into just such an issue plenty of times, so figured it was something like that.
If you don't mind, can you mark this thread as Answered? Thanks!