Board Thread:Question and Answer/@comment-27980822-20160323194336/@comment-25435121-20160323234804

The 4 million ATK is before the enemy defense is factored in as well as soldier loss on your cards. I believe Lis22 calculated that to tap for 999999 damage only requires your card to get to around 1.4 million ATK against a LAW. For AW/FAW it will be slightly less, for most map battles even less, and for Max level Limited Enemy, you will need a little more (around 1.5 million). The 4 million attack is really only possible with awoken burst as I believe it also raises your possible ATK cap against LAW. Without seeing the actual game source though, I can't be sure.

The limits are totals, so not maxing cards can be compensated by extra buffs as you suggest. Maxing cards just lets you hit that max first.