By Year 4 children should be quite familiar with the main metric units of measurement and should know the following:
1 kilometre = 1 000 metres
1 metre = 100 centimetres
1 metre = 1 000 millimetres
1 centimetre = 10 millimetres
1 kilogram = 1 000 grams
1 litre = 1 000 millimetres
It is a good idea to tell children that mille is Latin (and French) for 1 000, not one million!
This page is a good check of understanding this and getting children to use these units of measurement.