Me and a buddy were playing a game of VS the other night. He was using SF's with Mattie in it. Basically we had a debate over a play and rather than make him accept my ruling on it (the right one ;) ) I told him to leave everything as it is and we'll continue after the friendly people of VS Realms tell us how the card is played. There's two options, which one's right? Thanks in advance everyone!

Option A: I use Mattie's effect. I have 4 characters face down (A,B,C,D). I KO all four characters(A,B,C,D), which places four more characters(E,F,G,H). I can then KO those four (E,F,G,H) with Mattie's effect, all in the same attack. And so on until I decide I've KOed enough characters and stop.

Option B: I use Mattie's effect. I have 4 characters face down (A,B,C,D). I KO all four characters(A,B,C,D), which places four more characters(E,F,G,H). Since the characters aren't replaced until the effect (and attack) resolve, the largest number of character I could KO for that attack would be the initial four characters (A,B,C,D).

Mattie's effect triggers only once per instance of attacking or being attacked. If she is on the offense, the maximum ATK boost she'd get would be +4. If she was on the defense and attacked by a single character, it would also be +4. If team attacked, her effect would trigger once for each attacker, so it would be +4 * the number of attackers, potentially.

Dlanaan is correct. It only triggers once, and furthermore, the replacement occurs AFTER the bonus is given out. Option B is correct, unless there's a team attack she's defending against, in which case, A would be correct for a number of total iterations equal to the number of attackers she faced.

Just to be sure (though you seem to know what was going on), her effect actually resolves before the attack does. If her effect places attack oriented cards down (such as flying kick, savage beatdown, overload, etc), you will be able to use those before the attack resolves.

Actually, Mattie's effect will trigger for each attacker if she is team attacked. From the MSM Card FAQ:
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Mattie Franklin <> Spider-Woman, Gift of Power
Character, Spider-Friends, Recruit 6
11 ATK/11 DEF
Whenever Mattie Franklin attacks or is attacked, reveal any number of face-down resources you control. Mattie Franklin gets +1 ATK and +1 DEF this attack for each Spider-Friends character card you revealed this way. Then KO all Spider-Friends character cards you revealed, and put the top card of your deck into your resource row for each card KO'd this way.

If she is team attacked, this will trigger once per team attacker, and they resolve separately, and the bonuses accumulate, but you won't be able to use the same Spider-Friends character card for more than one of the triggered powers.
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So for each attacker, you would KO any face down SF characters cards and apply the bonus. If more SF characters cards were put into the resource row from the previous triggered effect, you can use them for the next triggered effect.

Sorry I should have been more clear that it was a solo attack. But you all answered the question regardless. Thanks!