A four-year battery lifespan is not bad these days.
Consider what loads your battery is subject to with all of the engine management systems and electrical accessories modern cars are equipped with.
I think my 300E got 6 years on the OEM unit, but we don't get severe winters in Texas, and the car is garaged otherwise.
Batteries are a wear item, and I'm not a purist when it comes to replacing them. Most forum members here would recommend Interstate products, especially the Optima (a pricey, but indestructible battery).
Truth is, as long as you match or exceed the CCA (cold cranking amps) and group (battery chassis size) and make sure it has the same type of terminals (on top versus side mount) as well as the orientation (+ on left, - on right or vice versa), any brand will do.
I have four cars to maintain...and the Bug takes two batteries, so I'm always looking to save money in this area...
Actually, picked up a couple of batteries for the Bug a month ago for $36 for both!!! Kmart is closing out their stores and had a fire sale!!!