Size 3-4 actually says size 4 on the tag and fits my 90th percentile almost 3.5 year old well as they were out of size 4-5 and the 5-6 was much too big. He is wearing a size four pant now. Im hoping these will last through the next six month season. Planning to buy a second pair for this price. Im a big fan of the holistic approach of making sure to get plenty of vitamin D so the more skin exposed, the better. These super cute trunks are perfect. Look great on a strong, healthy boy. Always admired this Euro style on young boys. Very cute.