Glenn is correct to wonder how much and what type of use is being described by "4.5 months".

However, I have read more than one hiker saying that the Merrell Moab's design emphasizes light weight, breathability, and comfort over durability. It's hard to make a light, breezy trail shoe for use in deserts that is as durable as shoes built for less hot places. You can try returning them as defective through the original seller, but I would not expect much different performance in a replacement pair.