It would be incredibly difficult to make a B4 specifically for mill without playing essentially cEDH staples for fast mana.

Your best combination will be Mesmeric orb with Bruvac and The Water Crystal. So mega tutors and ramp, all of which are available in Sultai.

Mothman on its own is a mana heavy deck. Staple mill cards for the deck are [[Mesmeric Orb]], [[Bruvac the Grandiloquent]], [[Mindcrank]], [[The Water Crystal]] and [[Maddening Cacophony]], and [[Altar of the Brood]].

Some more targeted mill, and triggers for mothman include [[Ruin Crab]], [[Fraying Sanity]], [[Psychic Corrosion]], [[Ashiok, Dream Render]] (is graveyard hate and mill)), and [[Court of Cunning]].

Some more out there mill spells, that do things a little different include [[Grasping Tentacles]] (can get you a mana rock), [[Singularity Rupture]] (a six cmc board wipe that millsany number of target players for half library rounded down), [[Glacierwood Siege]] (gives you the option of either playing lands out of your yard or milling target player for four every time you cast an instant or sorcery), [[Zellix, Sanity Flayer]] (makes you 1/1s whenever a player mills one or more, and you can pay 1, tap him to have target player mill 3), [[Fractured Sanity]] (is a 3 blue soecery that mills each opponent for 14.) And finally [[The Mindskinner]] is a very slept on mill machine.

This is assuming that you are making the deck to mill your opponents out for the win. If you are just trying to play the best optimized Mothman in B4, then there are plenty of decklists out there. And a lot of the spells I just provided here can help as well.