Newton's Puzzler Description

Newton's doctor ordered him to drink exactly four ounces of water at a time. He could not drink more. He could not drink less. He must drink exactly four ounces of water.

What's the Problem?

Poor Newton had no measuring cup. He had telescopes and prisms and all kinds of scientific equipment, but nothing in his lab to measure out the exact four ounces of water that he needed. Newton did have paper cups: he had the five-ounce size and the three-ounce size. He had no four-ounce paper cups.

How Can Newton Solve This Puzzler?

Newton is very smart. He filled one of the cups to its capacity and then...?

Can you figure out how Newton measured out exactly four ounces of water using only a three- and five-ounce cup?

Describe your solution. You can use all the water you need to solve the problem.