My soul is now 3 years and 1 month old. There have been no problems with the battery so far. However, I have had completely different experiences with battery life. From 3 years and 2 months to total failure at Nissan (zero goodwill). At Mitsubishi for up to 8 years. But I am like you think the 4 years should be in there. Based on my experience, I will replace my batteries prophylactically after 5 years.
Your failure rate is already blatant. There is a possibility that the Kia has bought a "bad" batch from the supplier. However, there could be some other technical defect.