Why not angular 3?

Because they wanted to sync the angular version with the angular router version. Angular will have a new version release every six months but Angular 4 will be backwards compatible with Angular 2

• Angular is being developed in a MonoRepo it means a single repo for everything. @angular/core, @angular/compiler, @angular/router etc are in the same repo and may have their own versions.
• The advantage of MonoRepo is, you don’t have to deal with the versioning of the code dependencies.
• Now the problem is with the @angular/router which is already in a 3.X version. And that’s because of some active and huge developments on the router section, like route-preload.
• Now releasing Angular as version 3, with it’s route on version 4 will create confusion
• To avoid this confusion they decided to skip the version 3 and release with version 4.0.0, so that every major dependency in the MonoRepo are on the right track.

ref :http://blog.angularjs.org/2016/12/ok-let-me-explain-its-going-to-be.html#Why_not_version_3_then_62