How to find previous releases of FortiClient VPN
From my experiences, I got stuck to find appropriate link to download version 6.4.0 through google search (may be my bad to do appropriate search).
But finally I found one solution that is interesting for me, and also want to share with you if you face same problem like me!
https://filestore.fortinet.com/forticlient/ there you should find list of available files to be downloaded, just use related path of your desired file with current url. For my case, it was https://filestore.fortinet.com/forticlient/downloads/FortiClientVPNOnlineInstaller_6.4.dmg