4 batteries in 4 years?!?

This does not sounds like a run of the mill 'doesn't get driven enough'...

Hard to say, but a parasitic drain from a module, door lock, etc, could drain the battery every night- the next day it is charged- but this drain/charge/drain/charge leads to a dead battery quickly.

It is diagnosable, just a PITA. Not a lethal condition.

FWIW...