What you'll want to do is place divs 2, 3, and 4 in their own container div (with the class .col-md-3) and 5 and 6 in another container div (with the class .col-md-3). Make div 1 have the .col-md-6 class.
Edit: You should use a media query to make it a fixed height in the desktop, then a flexible height when it's mobile.
@media screen and (max-width: 980px) { #div2 { height: 500px; (or whatever)}}