You can run v3.x and 4/5 if you deploy using a ColdFusion mapping to
reference the core framework:
http://docs.farcrycms.org/display/FCDEV40/Running+v3+and+v4+Together
One of the main features of FarCry 5.x is the ability to run the
entire framework under the specific webroot of your project. This
allows you to deploy different versions of FarCry 5+ for every single
project if that suits your environment ie. each webroot can have its
own farcry deployment.
Unfortunately, ColdFusion *always* sees the ColdFusion mapping before
it sees something under the webroot so this deployment approach will
not work with earlier versions of FarCry.
Long story short -- if you want to play with FarCry 5.1 deploy it
locally to a developer version of ColdFusion and see what you think.
-- geoff
http://www.daemon.com.au/