Overview
M4A1's Mod is one of the strongest mods in the game, as it opens up so much potential with the Cannon Mode. Her Skill 2 makes it so that when her Skill 1 is activated while 3 or less Dolls are on the field, she will pull out her Cannon and start blasting, dealing massively increased damage with an AoE at the cost of RoF.
The main thing to note with this is that using M4A1 in an Exodia comp dedicated to her Cannon Mode has much higher skill requirements than most normal echelons. You need to activate HG Skills before retreating them, and only then activating M4's Skill to let her start blasting. As such, using her for this purpose will require some decent kiting knowledge, for which you should
see the Kiting Basics page to learn more.
For farming, this obviously requires no more than 3 Dolls in the echelon to be AFKable, but any farming recommendations you see using M4A1 should already have taken this into account.
M4A1 can also obviously still be used in a normal ARSMG, as her tiles and Skill 1 both got some minor buffs from her Mod.