Where to Aim
As a general rule, armored vehicles have their strongest armor in the front and on the turret, with weaker armor on the sides, and the weakest armor on the top, bottom, and rear of the vehicle. For this reason, it's important to avoid taking shots - particularly with light anti-tank assets like the AT-4 - on the heavy armored parts of vehicles. Taking flank or rear shots is the best course of action, and occasionally you will even find yourself in a position where top or bottom shots become possible.
Good Anti-Tank Shots

Rear (L), Flank (R)
Bad Anti-Tank Shots

Frontal (L), Frontal Oblique (C), Rear Oblique (R)
UNITAF Standard Operating Proceedure (SOP) is adapted from two primary source materials - in addition to our own experience and past learnings:
US Army Techniques Publication, Infantry Platoon and Squad (ATP 3-21.8) ->view online
Dyslexi's Tactics, Techniques, & Procedures for Arma 3 (TTP3) -> view online