Required fields on tabs

Scenario: There are four tabs on a webpage. Each tab contains a webform. Some of the fields are required.

If a user doesn't complete a required field on tab two, and clicks tab four without saving, what should happen?

  1. The user is prevented from going to tab four until the required field is filled in
  2. The user is permitted to go to tab four, and tab two reverts to its previous information (all changes to tab two are discarded; there might even be a warning dialog before this happens)
  3. Something else

(I don't like putting webforms in tabs, but I've had to work on software that was set up this way.)