Consider a system of four uniform bricks, like in the sketch

https://imgur.com/EcnAWxV

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