All of the Angular modules were 2.x.x with exception to the router, which was version 3.x.x.

So to keep all of them at the same major version, they pushed everything to version 4.

Also, the jump from AngularJS (1) to Angular 2 was drastically different. Going from 2 to 4 however is much less significant in terms of breaking changes.

You won't have to relearn hardly anything if you already know 2.