Don't loop Ajax. If the server is overloaded, you will stack requests that will cancel each other

Instead do

function req() {
  $.ajax({ .......
    success: function() {
      setTimeout(req,5000)
    }
  })
}

If you want to send with 3 different intervals, you can do this

const timeouts = [4000,5000,6000];
let cnt = 0;
function req() {
  $.ajax({ .......
    success: function() {
      setTimeout(req,timouets[cnt]); // move this to an if to stop after 3 calls
      cnt++; if (cnt>= timeouts.length) {
        cnt = 0;
      }
    }
  })
}

You can send the request after every 5s using

time + time * (i - start)

function req(s) {
  console.log("After " + s + "s");
  // $.ajax({.......})
}

const start = 4000,
  end = 6000,
  time = 5000;

for (let i = start; i <= end; i++) {
  setTimeout(function () {
    req((time + time * (i - start)) / 1000); // I've passed the time to track the passed seconds
  }, time + time * (i - start));
}

You can take it as creating a series of time-lapsed requests via for loop.

function req() {
   console.log('requested');
}

for (let i = 1; i <= 10; i++) {
   setTimeout(req, i * 5000)   
}

However, such a series of requests can easily be achieved with setInterval too.

function req(){
   console.log('requested');
}

const limit = 10;
let counter = 1;

let handler = setInterval(function(){
   
   req();
   
   counter++;
   
   if (counter >= limit)
      clearInterval(handler);
   
}, 5000)