Fitbit introduced SpO2 tracking with its latest lineup of smartwatches, comprised of the Versa and the Sense, and the feature already made its way into the Ionic. Now blood oxygen saturation tracking is also coming to the much more affordable Fitbit Charge 4, along with a few other neat additions.
SpO2 tracking is part of the latest firmware update that's starting to roll out today. It allows you to use the Charge 4 to track your blood oxygen saturation overnight, and you'll be able to see the average on your wrist every morning. In the Fitbit app, you'll get some more stats, like your 7-day trend. The details are part of the new Health Metrics dashboard, available for the Charge 4 starting with the update. The dashboard gives you additional details on "health metrics like heart rate variability, RHR and breathing rate, along with your oxygen saturation (SpO2), and skin temperature variation."
The latter, skin temperature tracking, is another feature new to the Charge 4. Fitbit says that "understanding your skin temperature variations may help you identify changes to your body, such as the potential onset of a fever or ovulation."
The Health Metrics dashboard provides you an overview of your 7-day averages for free, but for anything beyond that, you'll need Fitbit Premium. Also note that Health Metrics and SpO2 aren't available in all markets, so it's possible you might not get these features after the update depending on where you live.