Price set for Golf Event
If you are using Drupal, I would guess you can do this with Webform-CiviCRM. For example, you might be able to use Webform Conditionals (which comes built in with Webform) so that if they select the "Single golfer" option on the payment amount it hides the other three contacts, and if they select "Foursome" it shows the additional contacts (and those fields can be marked required in the Webform Components section).