bump to v4.0.0
bump to v4.0.0

* dropping support for node < 4
* signal the CI not to test node < 4

TooTallNate/node-gyp,saper/node-gyp,TooTallNate/node-gyp,saper/node-gyp,refack/node-gyp,TooTallNate/node-gyp,saper/node-gyp,saper/node-gyp,TooTallNate/node-gyp,refack/node-gyp,refack/node-gyp,refack/node-gyp,saper/node-gyp,TooTallNate/node-gyp,refack/node-gyp