Can add attack damage cards stack 4 times without penalty?

[Navi]

i.e. +10 Knife[Andre Card x 4] does this add +80? or less?

[CrueZ]

It would add +80. There's no penalty for stacking of cards. Rather, there're bonuses for stacking different types of cards of the same + % damage.

For example, 4 Turtle General Cards would add +80% to your damage.

If you stack Hydra cards, which also does +20% but only to Demi-Human, to form a TTHH combination, the formula would become:

100% x 140% x 140% = 196% damage,

compared to the 180% if you simply stack 4 Turtle General cards together. The setback of TTHH is that the Hydra cards would only work for Demi-Humans.

[Aozora]

To put it simple;

Different +% Damage modifiers multiply while same +% Damage modifiers stack. Assuming that both cards effect the target monster.
+ATK cards only stack.