I love my IS-3, even with the crappy gun. The secret to killing an IS-3 head on is to target those hooks on the front. Maybe its a port, not hooks. I don't remember and I'm at work. I think I always shoot the driver side spot but I think there are two.
Sadly, I didn't realize this until I got my own IS-3 (paintball mod helps to see where hits landed). Now I can do some pretty serious damage to an IS-3 even in my little T29. I recently played peek-a-boo with a very bad IS-3 driver in my T29....and just annihilated him. I think I had 60% or more health left.
I usually try to shoot the flat sides of the pike, it goes through fairly reliably. On the rare occasion the guy's actually pointed at me straight, I used to shoot the center of the pike. Don't know if they ever fixed that little section yet, but back when I was playing it the center seam was the weakest part of the tank because it was completely flat and counted as unangled. Haven't tried it as of late though, don't see many Is-3s in my tier 6's. (it's mostly T-59s and american tanks right now)
But then I bounced 3 Bl-10 rounds off an is-3's sides yesterday, so....
The IS-4 is much much slower than the IS-3. If you like the speed on the IS3 you WILL be disappointed. It moves about as nimbly as the KV, but it has huge amounts of armor all over.
Just got my E-50 yesterday and I have to say.... this thing is sluggish!!! Turns like a slug and takes a long time to get up to speed. I'd say it's at least as nimble as the pII which is good, but it's much bigger and it's a juicy arty target as my first game showed...
Dumped free exp to get most upgrades immediately, the gun is beastly and freakishly accurate. I think I'll be playing this as a sniper/support gun platform as opposed to a brawler. I can't imagine trying to turn in this thing.
Good armor though, bounce 2 BL-10 rounds yesterday somehow.