how much ease on patterns – sail boat top

Hello, I've recently bought my first oliver +s pattern.. the sail boat set. I made a sail boat top for my little chap and it's really tight on him. He's quite a small 3.5 year old and I measured him up and he was right for a size 4. but when it was made up it was really tight across his tummy and the sleeves stop above his wrists. Can you recommend how much ease I should cater for in future when measureing him?? do I go for a size 6 next and give him a few extra inches or just one size up? thanks katy