Workout if we have managed to make 4.0/6.0 Backwards compatible

[KevinJump]

There where a few changes in the api between 7.5 -> 7.6 and 7.7 that necessitated the change in version number of uSync - however i think we might have mitigated these by backing the changes of into their own classes hidden behind version checks.

need to install and shakedown v4 on umbraco 7.4/7.5 because there is a distinct possiblity that it is ok and works fine. 🤕