having new field show up when particular radio button selection's made

Here's what I'm trying to accomplish in livecycle: I've looked all over this site, lynda.com and I just can't figure this out, though it seems common.

User can select from one of four radio buttons #1-4
If user selects #4, then another pair of radio buttons appear, where the user will then select one of them. If user selects #1-3, the new pair of buttons does not appear.

Any help or where I can find the answer to do this would be appreciated.

Mark