I've done some tests offline when the M4's first came out. The best way to kill a Tiger is to turret him from behind or the side (it will take 2 rounds from VERY close up, say under 100yds to be safe), and track or damage his engine, and then roll up JUST behind him, and put a round down on the engine deck (the M4 is tall enough to do so). You will want to be almost touching the back of his turret with your gun barrel. Aim down at the engine deck, and fire an AP round.
If the tiger is very busy (say there is a buch of tanks defending a Vbase and you get in behind the base with the tiger at the back), or if there are several tanks in close proximity (you won't want to waste multiple rounds and get killed before you kill the tiger), you can do this without turreting him or damageing his engine.