despite offering free postage ,buyer opened a dispute when he didn't get it 4 days after buying. This is despite it saying 3-4 working days postage and the 4th day being a Saturday.
Just because you offer free postage doesn't mean you can send it out whenever you feel like it. You still should get it there within the delivery estimate you gave. Saturdays count as delivery days so if you selected 3-4 days postage it would have shown the estimated delivery as being the saturday:
Still, he's being unreasonable and you should call eBay as others have suggested to sort it out.