If your cars battery is 4 years old or older you need to replace the battery soon because 4 years is pretty old for a battery.
When your battery is going bad and needs replaced your engine may crank over slowly, check engine light may even come on, or your battery might start swelling out.
If your battery is doing any of the above or is 4 years old or older you need to replace it before it dies completely on you and leaves you stranded.