First try restarting Firefox and emptying the trash. If that doesn't help, if all local changes you've made have been uploaded, you can do a Restore from Zotero Server in the Sync -> Reset pane of the prefs, which will replace your local data in all libraries with data from the server. (Don't use any of the other options.)
If there are local changes you need, your best bet is probably to install the 5.0 Beta (which will be released as 5.0 Final soon, so it should be pretty stable). Bugs in 4.0 can't be fixed at this point, but 5.0 might fix this, and we can troubleshoot this further in 5.0 if necessary.
If you do upgrade to 5.0 Beta, you can switch back to the official release once 5.0 is out.
Upgrade Storage
If there are local changes you need, your best bet is probably to install the 5.0 Beta (which will be released as 5.0 Final soon, so it should be pretty stable). Bugs in 4.0 can't be fixed at this point, but 5.0 might fix this, and we can troubleshoot this further in 5.0 if necessary.
If you do upgrade to 5.0 Beta, you can switch back to the official release once 5.0 is out.