Some schools require that you be on the school's network in order to submit electronically, so you may need to be on campus for Exam4's network connection to work.
It is also possible your security software (such as Norton Internet Security) is blocking the transmission. Try disabling this for a brief time and attempt to submit the exam while it is turned off.
In some rare cases with macOS this can be caused by Safari's Caching mechanism. If you are on a Mac, try clearing the browsing history in Safari:
> Launch Safari
> From the Safari Menu choose "Clear History..."
> Select "all history" from the "Clear" drop down box, then click "Clear History"