If you don't strictly need 6.4.6, download the latest VPN-only client directly from Fortinet's official public page (no login required for latest version):
Runs efficiently on older versions of Windows and macOS without performance degradation. forticlient vpn 6.4.6 download
Official link for FortiClient VPN offline installer? : r/fortinet If you don't strictly need 6
FortiClient VPN 6.4.6: Comprehensive Download and Setup Guide If you don't strictly need 6.4.6
: For environments without direct internet access, the full offline installer can be retrieved from the same firmware images section on the Support Portal. Installation Steps