According to this, 4.5 *should* work for the Enhanced Google Mail Plug-in
BlackBerry - Enhanced Gmailxxx8482; Plug-In
Doh! that probably wasn't what you were looking for... Sorry.
This may be enlightening.
KB19269-What is Google Mail contact synchronization
Note: Two-way wireless contact synchronization with the Google Mail contact list requires BlackBerry Device Software 5.0. Synchronization must be configured through email settings on the BlackBerry smartphone. Additional requirements and constraints may be found within the BlackBerry Internet Service 2.8 documentation kit. This feature is not supported on BlackBerry smartphones with BlackBerry® Connectxxx8482; software.
Note: Google Mail contact synchronization is not supported on versions of BlackBerry Device Software earlier than version 5.0.284.