What reference tells you that AnyConnect 4.x is only supported on the 5500-X series?
The following AnyConnect 4.1 document states otherwise:
http://www.cisco.com/c/en/us/td/docs/security/vpn_client/anyconnect/anyconnect41/feature/guide/anyconnect41features.html
...as it show many features are compatible with ASA software as old as 8.0. (Note X series hardware support was only introduced as of ASA software 8.6(1).)
There are some newer features that won't be supported on the older hardware as the non-X-series is only supported up through ASA software 9.2