AFAIK, 4 of the same cards can be stack together but the damage output is not that big compare to a different combination of cards..
As I recall,
If you compound 4 Turtle general cards in a weapon then the damage modifier will be something like this.
1.80 or 80%
while compounding 2 TG and 2 Hydra will be..
1.4 x 1.4 = 1.96 or 96%
I read it somewhere. Correct me if im wrong.