Bricks and rotation

Sep 2017
Hey, there is a problem I don't really know how to approach :) it goes:
Consider a system of four uniform bricks, like in the sketch
Each brick has
length = L
height = H
depth = D
What is the maximum distance x that the middle two bricks can be displaced outward before they begin to tip?
So far I was thinking of trying to figure out how much the middle bricks can move outward and still have an angular acceleration \(\displaystyle \alpha = 0\), but I'm not sure if that's a good approach or how to proceed (my skills in physics are rusty). Thank you in advance :)