Too many variables to discuss "an almost 4 year old battery". Doesn't means the manufacturer makes bad batteries........
Not to mention the fact the OP doesn't even own the vehicle that has the battery in it.
Overall I agree with your thought process here.
There are a lot of variables that could be unknown. In this particular case they are known by Walterjay.
I had a AZ AGM battery that lasted 3 years 8 months… I was not impressed. Car driven every single day… And for longer distances… 28 miles on way and back.
And… it spent the vast majority of its life in an old 98 Camry. . NOT exactly bristling with lots of parasitic electronics…
My theory… why batteries are not lasting as long…
1) obviously much much more parasitic draw on car batteries now when not running… and keyless fobs keeping the cars using more power of key is close enough.
2) the whoopty doo fuel saving strategy utilized in vehicles built after the mid 2000s.. Not helping to keep car batteries charged better. 80-85 percent is about all they do for typical starter batteries… Which is the real killer of lead acid and AGM batteries. They live by far longer when truly kept near 100 percent charge after each use.
3) the use of smaller and smaller lighter batteries in all vehicles… the Nissan I had specd a group 35. Yet the same exact motor specd a group 24f just a couple years before. And yes there can be a large weight difference between those batteries. A solid group 24f can weigh 44 pounds and a group 35 can weigh as little as 33 pounds. Mind you a Northstar group 24f weighs nearly 58 pounds… Exact same BCI size. Which gives an indication of how much less lead is being utilized in newer and newer batteries.
4)The percentage of good pure lead in batteries has and continues to driop which affects the actual build quality all those batteries and their shorter longevity .