Sheesh. The important factor is not round or flat, it's penetration angle, as stephen implies. A rounded turret, if hit at the point where the circumference is perpendicular is 90 degree penetration, i.e. same as if it were flat, vertical, and perpendicular to the incoming round. Thus, when aiming at the side of the rounded turret, make sure to hit the center, both left right and up down, i.e. where the metal is 90 deg/ 90 deg (both axes) to your incoming round. Or, forget the turret and hit the vertical side of the hull (not the tracks).
The M4 mantlet is very thick- never hit the turret if the mantlet is pointed at you. Also, the glacis is sloped so don't hit that either. Hit the vertical center of the rounded armor below the glacis.* You can take out an M4 in one shot from a T34/76 if you hit there at ranges under 1K.
Penetration angle is what is important. Although penetration is much more complex than this, just think of the thickness of the armor along the path of the incoming round. If the armor is 75 mm thick and at 90/90 degrees to the incoming round, it has to go through 75 mm. If the same armor is at 45 degrees then it has to go through square root of 2 = 1.41 times as much or 1.41*75 = 106 mm.
*this may actually be a bug. I don't know about this armor in an M4. However, this technique also works with the T34 because, I guess, the armor is modeled there as the same thickness as the glacis but at a 90 degree angle (ie vertical). The actual T34 had an enormously thick casting at that point, so penetrating there is not quite 100% realistic.
