[chrisco255]

Yes. We built out a lot of NG1 components at my company and are currently building a product in NG4. The issues are, for example, the template syntax is completely different. That has to be mapped over. There is no $scope and the component lifecycle is completely different in NG4. Depending on how you stored state, responded to state, and depending on which ngDirectives you used...there may or may not be a direct mapping with concepts for NG4. Also, if you have any testing in place at all, those tests will likely need to be rewritten.

NG4 components depend on RxJS, for example...but Angular 1 had no such concept. SystemJS modules are a thing in Angular 4...but not in Angular 1. You may or may not have ES6 modules in your Angular 1 code base...but you definitely have them in your Angular 4 code base. There are so many differences that "upgrading" to Angular 4 is really just a rewrite in practice. And it's no less effort for you to migrate to React or Vue in a piecewise manner.