I Myself have found 3 frontal weak points on a IS-4.
the Front Drivers Hatch.
the front side slabs when they try to angle the front driver hatch
and..
the lower hull. there is a track piece bolted to the lower front this area is the spot. also as an extention the corner where the upper and lower meet is an extention of this weak point.(this is common on all tanks with a lower hull weakspot like the Tiger 2 etc), Its the exact thickness of the front (160) and not angled enough to bounce.
I've blown holes in the front of Is-4s with my 105 (T29/32) much to thier surprise, and ripped them open with a 155 round (T95), several times even ammo racking them in 1 shot. my Kt aims for the same spot as well as my Lowe with teh same results, damage to the Is-4 amd even the Is-4s death.
Its hard to hit from range but with accurate guns it can be done (and has been done.)