const source$ = Rx.Observable.of(1,2,3,4)

const job$ = source$
  .do(x=>console.log("emitted from source " + x))
   .switchMap(n => fakeAsyncWork(n))
  .do(x=>console.log("emitted from switch " + x))
  .scan((a,c) => a.concat(c), [])


let fakeAsyncWork = (sec): Rx.Observable<any> => {
  return  new Rx.Observable(observer => {
          console.log("start waiting for " + sec);
          setTimeout(() => {
                console.log("finish waiting " + sec);
                observer.next(sec)
            }, sec*1000);

  });
}

job$.subscribe(x => console.log(x) );

"emitted from source 1"
"start waiting for 1"
"emitted from source 2"
"start waiting for 2"
"emitted from source 3"
"start waiting for 3"
"emitted from source 4"
"start waiting for 4"
"finish waiting 1"
"finish waiting 2"
"finish waiting 3"
"finish waiting 4"
"emitted from switch 4"
[4]