Description:
If you deactivate Payeezy for WHMCS in Setup > Payments > Payment Gateways and get a white screen with:

Gateway Module "firstdatae4" Not Activated

when trying to view invoices.

Root Cause:
This is caused by the hook file located in /path-to-whmcs/includes/hooks/hooks_firstdatae4.php
The hooks tried to run even when module has been deactivated.

Workaround:
A temporary solution is to comment out the last line in the hook file (by adding two forward slashes // in front) like this:

//add_hook('ClientAreaPageViewInvoice', 1, 'hook_template_variables_firstdatae4');

Permanent Solution:
Remove the hooks_firstdatae4.php from /path-to-whmcs/includes/hooks/ when deactivating the module.

This will prevent the hook from running when the module isn't active.