how to make responsive nested row and column in bootstrap code example

Example: nested columsn inside bootstrap

<div class="container-fluid">
  <div class="col-lg-12">
      <div class="row">
        <div class="col-sm-6">
            <div class="well">
              <p>First Part</p>
            </div>
        </div>
        <div class="col-sm-6">
            <div class="well">
              <p>Second Part</p>
            </div>
        </div>
      </div>
        <div class="row">
            <div class="col-sm-6">
              <div class="well">
                <div class="row">
                  <div class="col-md-6">
                    <div class="well">
                    <p>Nested 1</p>
                  </div>
                  </div>
                  <div class="col-md-6">
                    <div class="well">
                    <p>Nested 2</p>
                  </div>
                  </div>
                </div>
                <div class="row">
                  <div class="col-md-6">
                    <div class="well">
                    <p>Nested 3</p>
                  </div>
                  </div>
                  <div class="col-md-6">
                    <div class="well">
                    <p>Nested 4</p>
                  </div>
                  </div>
                </div>
            </div>

            </div>
            <div class="col-sm-6">
              <div class="well">
                <p>Fourth Part[enter image description here][1]</p>
                </div>
              </div>
          </div>
    </div>                      
</div>