Parry...

[ZDamned]

I was told at some point while leveling a paladin that parry doesn't work if a mob is behind you or on your flank. That said, nearly every tank I've run T4 with has let themselves become completely surrounded every chance they got. So, if the tip about parry is correct, and if the OP tanks T4 the way I've seen it done so often in the past, then I'd say no conclusion could be drawn from this test at all.
If the tip is wrong, I'd like to know cause I bust my @ss all the time trying to keep all mobs in front of me and wouldn't mind quitting that.

I tested this yesterday, and it is true, you can NOT block, OR parry anything behind you or too far to your side.
ALSO!
I do NOT tank T4 like a moron. I in fact hold everything in front of me, and did so for each of my runs.