How fast do kids gain weight?

[cleareyes]

My little bean is 4years old (and 5 months)
He is 1lb lighter than when he was 4 so is dropping down the centiles
He's only short, he's now 15th for weight and 3rd for height.
Is this normal? Will he just have a sudden growth spurt at some point?

No, loosing weight isn't normal.
Kids should always be getting heavier.
Growth spurts involve getting much taller at a similar weight - so getting thinner - but not lighter.
If you are sure about the numbers, and don't have a reason like illness to explain the loss, GP appointment.