four years is a long time and you can't always see damage. Some string materials do not frey, some do and it means nothing at times. Also, most strings break under the serving where you can't get wax to them.

Also depends on how often you shoot, how "nice" you are (do you drag your bow through brush alot or sit in tree stands), dusty dry weather, etc etc

In any case, 4 years is a while.