4
+ 1 turn + 2 cards for 6|2 with double benefits from attack buffs and less dr.
4
+ 1 turn + 3 cards for 6|3 with double benefits from attack buffs and less dr.
Your card
5
+ 1 card for 6|4 with double benefits from attacks and also chance for extra attack.
Yours is cheaper and better than the two above examples.
And upped is just OP. We've seen many many examples of triple attack cards and all of them were shot down for the same reason. Triple multiplier would be way too much when combined with buffs.