My 5.5 year old son weighs about 39.5 pounds and is 42.5 inches tall. A size 4 should be perfect for him as the sizing chart lists a weight range of 37.5 to 42 pounds and a height range of 42 to 44 inches. The only thing I would say fit him well was the vest. The shirt was a little big but bearable. Pants were huge! Yes, there is an adjustable waistband, but oh well! These are pants from a suit; They shouldn't fit like a sack of potatoes. The cut of the jacket was also strange. The shoulders were much wider than my son's actual shoulders, and it just looked weird with the (non-removable) shoulder pads. The suit jacket wasn't straight either; Maybe it was a bit narrower at the waist? I don't know what happened there. The fit of this suit was just odd. The quality of the costume was ok. Not cheap, but not effective either. For reference, I usually buy my son 5/5T shirts and 4T pants (sometimes 5/5T pants if I can tell he needs 5T and I can adjust the belt).
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