To complicate matters, not all iron should be considered equal per units mass, some forms of iron are considerably more bio-available, and in general the forms provided as supplement on cereals in are on the low end of bio-availability.
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).