Major Release 4.0.0

NOTE: "Major" here used loosely. This release is an interim major release that introduces ES6 syntax into the package without changing the import method (require()). There will be a follow-up major release that switches entirely to ESM and will set the appropriate engine key in package.json. This was just a necessary first step to allow those who don't (yet) use ESModules a sane place to upgrade and pin to.

• Move ES6 syntax (i.e. const/let, still using require)