v4.0 Item Stacking is very different from v3.0. In 3.0, items usually did not stack, as they would essentially double the effects of the stacked items, which in many cases was far too powerful.
In v4.0, diminishing returns was introduced. This system essentially dumbs down the item effects from multiple stacked items with an exponentially decreasing system.
Items with "subject to diminishing returns" in their description follow a diminishing multiplier to their affects:
As you can see, there isn't much of a point in stacking an item more than 3 times, as the bonus is reduced to only .4 of the listed bonuses for the item on the 4th stack.
Spell Resist and EvasionEdit
Spell Resist and evasion follow their own system of Diminishing Returns.
Lets use a pretty basic example: The Spell Buffer's spell resist:
The Spell Buffer gives +26% spell resist. With diminishing returns, this does not mean that with two spell buffers, you get 52% spell resist (26*2=52). What diminishing returns does is take each application of a stacked item into account individually.
In v3.0, with the old stacking system, two spell buffers would give you 52% spell resist, reducing a 100 damage spell to 48 damage.
In v4.0, if you were hit by a 100 spell damage attack with 2 spell buffers, the first spell buffer would reduce this by 26% to 74 damage. THEN the second spell buffer's resist comes into effect, diminishing the 74 damage by an additional 26%, reducing it to 54.76 (rounded to 55) damage.