Looks like Charge 5 to me. 3rd party chargers from Amazon for less than $10 wouldn't cost that much to try, unless you know someone with a Charge 5 you can try out their charger to test.
Hello @LindaRichards and welcome to the Community. I think that is a Charge 5. That metallic ring on the back side looks like the ECG sensor. The pebble has very rounded edges. Both the Charge 3 and 4 have squared off edges.
All three models have different chargers, so it's good that you came here to ask.
Laurie | Maryland
Sense 2, Luxe, Aria 2 | iOS | Mac OS
Take a look at the Fitbit help site for further assistance and information.