i would be very afraid of trying to be too clever with such things. people using varnish should know more or less what they are doing, or read the doc that explains the diff between 3 and 4. and i could use varnish 3 but add some vcl to use no-cache, or vcl in varnish 4 to ignore it because i look at another header and use no-cache for the world outside varnish only.

as this configuration is not things done on reusable bundles, i don't think we need to support several caches in parallel - you can adjust the configuration if needed when you upgrade varnish.