sand is deep, your answer is not quite correct. from what i understand, you have exactly 4 stones, each of a different weight. so you cannot keep four 1kg "types" to weight 4kg's. from kiwiland's answer is the best - don't know if there is another answer. with his approach, you can measure all weights up to 40 using just those 4 stones, PROVIDED you are using a mass balance(taraju). for example, to measure 2kg's, you keep the one 3kg stone on one side and the one 1kg on the side of the rice. using various combinations, you should be able to measure up to 40 kg's.