Untitled

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title></title>
    <style media="screen">
    input[type='number'] {
      appearance: textfield;
    }

    input[type='number']::-webkit-inner-spin-button,
    input[type='number']::-webkit-outer-spin-button,
    input[type='number']:hover::-webkit-inner-spin-button,
    input[type='number']:hover::-webkit-outer-spin-button {
      -webkit-appearance: none;
      margin: 0;
    }
    </style>
  </head>
  <body>
    <form id="form">

    </form>
  </body>
  <script type="text/javascript">
    const form = document.getElementById('form');
    form.addEventListener('submit', (e) => {
      e.preventDefault();

      let number = '';
      const numbers = document.getElementsByTagName('input');

      for (let nr of numbers) {
        number += nr.value;
      }

      alert('Submitted ' + number);
    });

    function buildForm() {
      for (let i = 0; i < 4; i++) {
        const numberInput = document.createElement('input');
        numberInput.setAttribute('type', 'number');
        numberInput.setAttribute('id', i);
        numberInput.setAttribute('max', '9999');
        numberInput.setAttribute('maxlength', '4');
        numberInput.addEventListener('keydown', (e) => {
          handleInput(e, i);
        });
        if (i === 0) {
          numberInput.value = 3528;
          numberInput.setAttribute('disabled', true);
        }
        form.appendChild(numberInput);
      }
      const submitBtn = document.createElement('button');
      submitBtn.setAttribute('type', 'submit');
      submitBtn.appendChild(document.createTextNode('Submit'));
      form.appendChild(submitBtn);
    }

    function handleInput(e, i) {
      const input = document.getElementById(i);
      if (e.keyCode !== 8 && input.value.length === 4) {
        if (document.getElementById(i+1) !== null) {
          document.getElementById(i+1).focus();
        } else {
          document.getElementsByTagName('button')[0].focus();
        }
      } else if (input.value.length > 4) {
        input.value = input.value.slice(0, 4);
      } else if (e.keyCode === 8 && input.value.length === 0) {
        document.getElementById(i-1).focus();
      }
    }

    buildForm();
  </script>
</html>