The Aranet 4 claims it can measure up to 5000 ppm with +-3% accuracy. But you'd have to take their word for it, I guess. For $250 I'm hoping it's not a crap sensor.
The usual convention is that the accuracy refers to full-scale measurements. I.e., your device has an error of +-3%*5000ppm = +-150ppm. At ~400ppm you are about 37% off.
Human exhaust breath contains about 5000ppm CO2, so this device is decent for measuring humans. It's less decent to measure atmospheric CO2.
Edit: looking at the datasheet, the device claims +-30ppm and 3% of reading, which I interpret as "whichever is greater". Thus, the device would be +-30ppm up to 1000ppm, and 3% of the reading above 1000ppm.
Human exhaust breath contains about 5000ppm CO2, so this device is decent for measuring humans. It's less decent to measure atmospheric CO2.
Edit: looking at the datasheet, the device claims +-30ppm and 3% of reading, which I interpret as "whichever is greater". Thus, the device would be +-30ppm up to 1000ppm, and 3% of the reading above 1000ppm.