Unfortunately, it seems to be a common problem with the charge 4. Mine broke in that same piece 3 different times. Fitbit will replace it until it’s out of warranty, you just have to reach out to them, which you can do through your app- it’ll connect you via text or have someone call you once you report the problem.
Note- you can’t get just the connection piece, it has to be the entire pebble. Once it’s past the warranty, they offer a discount on a new one. BUT you can get a replacement band that houses the entire pebble- it doesn’t need the connections. There are options on on Amazon for $15 or less, depending on what band you like. My daughter likes this one:GOSETH Compatible with Fitbit Charge 4/Fitbit Charge 3 Bands with Case, Silicone Strap with Shatter-Resistant Protective Frame for Fitbit Charge 3/SE/Charge 4 and Special Editions (Black&Blue). I personally like this one: Sport Band Compatible with Fitbit Charge 5 Band with Case for Soft Silicone Strap with Shatter-Resistant Protective for Fitbit Charge 5/ Charge 4/Charge 3 Advanced Fitness Tracker. No sense in getting a new one if the pebble itself still works and you’re past the warranty period.
Hope this helps. TLT
