If the car has sat idle during the work week for 4 yrs and you got 4 yrs. out of the battery, you did quite well. I used to get long life out of car and motorcycle batteries, but in the last 10 yrs. or so, it seems that 3-4 yrs. is about as good as your going to get unless you buy one of the highly touted Optima's.

One thing's for sure. Your charging system is doing it's job and you have little or no battery drain.