Often when we go to a store to buy clothes we try them on, yet when buying clothes for our children we rarely get them to do so. When they are babies or even toddlers, it can be a challenge getting them to try items on so therefore we often just buy the size we think they need, but as they get older, buying clothes based on their age alone can prove to be inaccurate.
I have a child that is 3 and currently in to age 4-5 if not 5-6 age clothes as she is so tall, so going out and buying age 3-4 would simply be a waste of time and money. When shopping for clothes for children be sure to look at the height or waist measurements too as these are often a better indication of which size they need than based on age alone. Ideally take a few simple measurements of your child every few months so you have a rough idea when you do make your next purchase.