Rx buffering and flow control

const Observable = Rx.Observable;

let input1$ = Observable.fromEvent(document.querySelector('#b1'), 'click').mapTo('1');
let input2$ = Observable.fromEvent(document.querySelector('#b2'), 'click').mapTo('2');
let input3$ = Observable.fromEvent(document.querySelector('#b3'), 'click').mapTo('3');
let input4$ = Observable.fromEvent(document.querySelector('#b4'), 'click').mapTo('4');

let inputs$ = Observable.merge(input1$ ,input2$ ,input3$ ,input4$);

function getPromise(x) {
  console.log('start promise');
  return new Promise(resolve => {
      setTimeout(()=>{
        console.log('resolve promise');
        resolve(x);
      }, 5000);
});};
/*
four buttons generates events that should be buffered
and send to a remote server, howevere the server can
avccept only one request per time
*/
inputs$
  .buffer(inputs$.throttleTime(2000))
  .filter(x=>x.length>0)
  .concatMap(x=>{ 
        let p = getPromise(x); 
        return Observable.fromPromise(p) 
   })
  .subscribe(x=>{console.log(x);});