You have to weigh every unit of kilograms from 1 to 40 inclusive

You have to use 4 weights

The total weight of the weights is 40 kilograms

The scales are the type shown here

What are the four weights?

You can use the weights in combination on either sides of the scales

eg. weights of 5 and 7 can weigh 2, 5, 7 and 12 kilos etc.