UK - Add any 4 posters to your basket and when you go to pay it will only charge you for 2 posters. 

Worldwide - Add 4 items to the basket and then request an invoice where I will adjust it so you only pay for 2 items and pay postage only once. If you can’t find the ‘request an invoice’ button you could always purchase 4 items as normal and then I would refund the costs.