What is Angular 4 and from where I can learn more about it?

Angular 4 is just a newer version of Angular than Angular 2.

Angular 2.x.x, and Angular 4.x.x are also named Angular, while the previous version (1.x.x) is named AngularJS.
This name difference indicates that there are major differences, while Angular 4.x.x is only a newer version of Angular than Angular 2.x.x is.

Angular uses semantic versioning since Angular 2, which requires the major number being increased when breaking changes were introduced.

The Angular team postponed features that cause breaking changes, which will be released with Angular 4.

Angular version 3.x.x was skipped to be able to align the version numbers of the core modules, because the Router already had version 3.

To see what changes will be introduced in Angular4 see https://github.com/angular/angular/blob/master/CHANGELOG.md

See also this blog post http://angularjs.blogspot.co.at/2016/10/versioning-and-releasing-angular.html

1)What’s New?

1.1 Smaller & Faster

1.2 Animation Package

Pulled animations out of @angular/core and into their own package. This means that if you don’t use animations, this extra code will not end up in your production bundles.

2) New Features

2.0) View engine

The new view engine significantly improves the compilation step which results in smaller application size compared to earlier iterations.

2.1 Improved *ngIf and *ngFor You can now use an if/else style syntax, and assign local variables such as when unrolling an observable.

  <div *ngIf="userList | async as users; else loading">
   <user-profile *ngFor="let user of users; count as count" [user]="user">
   </user-profile>
   <div>{{count}} total users</div>
 </div>
 <ng-template #loading>Loading...</ng-template>

2.2 Angular Universal

2.3 TypeScript 2.1 and 2.2 compatibility

2.4 Source Maps for Templates

Now when there is an error caused by something in one of your templates, it generate source maps that give a meaningful context in terms of the original template.

3) Packaging Changes

3.1 Flat ES Modules (Flat ESM / FESM)

This format should help tree-shaking, help reduce the size of your generated bundles, and speed up build, transpilation, and loading in the browser in certain scenarios.

3.2 Experimental ES2015 Builds

This option is experimental and opt-in. Developers have reported up to 7% bundle size savings when combining these packages with Rollup.

3.3 Experimental Closure Compatibility

making it possible to take advantage of advanced Closure optimizations, resulting in smaller bundle sizes and better tree shaking.

I got this information from this blog...... for more information refer here