The age statement on a whisky label tells you the age of the youngest whisky in the bottle. What does this mean? Put simply, a bottle of whisky may contain whiskies matured in a number of different casks for different periods of time. If the label says that the whisky is 4 Years Old (or Four Years Old) then, although it may contain older whiskies, you can be certain that none of components are any younger than 4 years.
Once a whisky is bottled it ceases its maturation, unlike wine which continues to age in the bottle, so Four year old whisky is frozen in time and will be considered 4 forever.