To get the weather to sync on your Charge 4, the weather app must be installed, and the relevant permissions allowed....
Saying as the weather won't sync, I'm assuming that the weather app has been installed from the gallery. First thing I'd do is to check the permissions.
For this, in the Fitbit app, tap on your account button (at the top left, to the left of the fitbit logo)
Then select/tap your Charge 4
The select/tap Gallery
Then select/tap Apps at the top of the page
Then select/tap Weather
Then select/tap Permissions
Hit Select All
And sync your phone/Charge 4. Once that's done, go to the weather on the Charge 4 and it should display.the weather for your current location
If the weather app still isn't showing any data, I'd recommend uninstalling the weather app and reinstalling it. So we do the above again, up to and including the select/tap on the weather app.
Now tap the - uninstall button. The fitbit app will tell you that it'll remove all of the data associated with the app, but that's fine - so tap uninstall to confirm. After a short while, the fitbit app will then show a + install (and the weather app/option will now no longer be on your Charge 4)
Now tap on install. You'll then get another window up regarding the required permissions. Tap on 'Select All' and then 'Proceed. your Charge 4 screen will go blank, apart from a horizontal progress line. Once it is installed, you should see a check/tick mark on your Charge 4 screen, accompanied by a little bit of vibration.
hit the back arrow at the top left in the app to get back to the main display, then force a sync between your phone and your Charge 4. This should then show you what AccuWeather believes the conditions are for where you are 🙂
