If you receive the message "Failed to sign in. Please try again. (Error Code: 4)" then there is an issue with your Zoom application's local data.
The simplest way to resolve this issue is to uninstall the Zoom application, then download and reinstall it.
Alternatively, on a Windows device you can open File Explorer and locate the file named %appdata%/Zoom/data then delete it.
On a MacOS device open Finder and locate the file named ~/Library/Application\ Support/zoom.us/data then delete it.