although a bit late for my reply, but 4.5 years is about average time the batteries last, each battery can perform differently as it depends on number of charge and discharge cycling and so if your wife's car is only used for smaller shopping trips and collecting school children, then she is performing far too many start stop cycles, as opposed to your v6 battery that is large (heavy duty) to begin with and probably has less starts per day.
It is not uncommon for a new battery to last just 3 years before it looses its capacity to store charge, even if the terminal voltage indicates a healthy battery, it is the storage capacity that suffers.
It is not uncommon for a new battery to last just 3 years before it looses its capacity to store charge, even if the terminal voltage indicates a healthy battery, it is the storage capacity that suffers.