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ok hi here's a comparison between 4 pc wanderer's and 4 pc crimson witch for yanfei based on my calculations:
wanderer's 2 pc effect is 80 elem mastery, which increases the dmg caused by melt/vaporize by 15%, which is equal to one of the effects in 4 pc cw, increasing melt/vapo dmg by 15%
4 pc wanderers boosts catalyst/bow charged attack dmg by 35%, however at full stacks, 4 pc crimson witch can reach 37.5% pyro dmg bonus
which, yanfei can maximize bc her skill cd is 6s
plus 4 pc cw increases overload and burning dmg by 40%, and also covers her normal atk, skill and burst dmg, not just her charged atk so
imo i'd go for 4 pc cw but if you already have a good set, whether or not it's wanderers or crimson witch, go and use it
ofc all of this is naught if my math is wrong so if anyone can correct me pls do 
