We would suggest writing a letter of complaint to the provider of the four year warranty, whether this is the supplier or the manufacturer, asking for an explanation as to why they are refusing to honour the warranty. It is also worth bearing in mind that if you car is equipped with a stop-start system it is possible that the new battery would need to be coded to the vehicle, this is so the vehicle can alter the operation of the alternator for a new battery as opposed to an older one.