From merchant website:
Simply purchase 3 tyres from the Continental Passenger or 4x4 ranges to receive the 4th for FREE!
Clarification
Haven't confirmed with JAX customer service, but if you proceed through their online checkout, this is how it goes down:
You get the 4th tyre for free, but you still have to pay $15 to get the (4th and free) tyre fitted.
So the free part is just the tyre itself, not the other charges associated with buying a single tyre.
Total price ($) = 3 * continental_single_tyre_price + 15
Then you get the 5th for free.