The Spice Merchant's Weights

The Story: In the old spice market of Kochi, a merchant named Ibrahim has run his cardamom stall for forty years. He uses a traditional balance scale and a small set of brass weights to measure out purchases for his customers. One afternoon, a young mathematics teacher named Kavitha stops at his stall and watches him work. She notices he has only four brass weights but seems to be able to measure any integer number of grams from 1 gram all the way to 40 grams. 'How is that possible with only four weights?' she asks. Ibrahim smiles. 'I do not just put the weights on one side,' he says. 'I put them on both sides.' Kavitha's eyes light up. She immediately understands. She asks: 'What are the four values of your weights?' The Challenge: What are the four weight values that allow Ibrahim to measure every integer from 1 to 40 grams using a balance scale, where weights can be placed on either pan?