Hi,
I think the Web deployment would be a good idea as it will prompt all the users to upgrade the client to 4.x.
Here is a link for the same:
http://www.cisco.com/c/en/us/td/docs/security/vpn_client/anyconnect/anyconnect40/administration/guide/b_AnyConnect_Administrator_Guide_4-0/deploy-anyconnect.html
Regards,
Aditya
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