By wanting to learn Angular you're definitely riding against the ecosystem, as shown in the post, see: http://stateofjs.com/2017/front-end/results and note that most do not want to use it/or have used it and would not use it again. (Myself I am in the would not use it again camp). What makes you interested in Angular as opposed to other front end frameworks?

Angular 4/5 is simply the continuation of Angular 2, so it's more like 2/4/5. The Angular router had been up to a major version 3 so they skipped Angular 3 as a whole in the interest of keeping all of the parts of Angular in version sync.

Starting with the latest version of Angular is generally the best idea, though you may be tied to earlier versions depending on what other modules you want to use. For example, the npm version of ng-bootstrap is still tied to Angular 4 (though one can easily build one that does from the github repo). I would recommend starting with ng-cli as most (older) books will start you off with some other build system (e.g., system-js, an older webpack plugin).

It depends what you already know.

If you don't know a ton of other frameworks, tools and the like AND need to get something built this quarter, learn the latest version of Angular 1.x

Otherwise, learn Angular 2, Typescript and all the other stuff that people use for building etc. all at once, focusing heavily on Typescript and core Angular concepts first.