Question: what is the difference between the fitbit 3 and 4?
Answer: Sensors. The Charge 3 already had an array of powerful sensors, and the Charge 4 only builds on it. The Charge 4 makes NFC standard for Fitbit Pay (previously it was limited to the SE model) and also adds built-in GPS, a first for a Fitbit tracker. Both models are also swimproof up to 50 meters.