Winter Soldier and KM synergy

I have a 5* KM and a 4* Winter Soldier. Should I rank up Winter Soldier, to what rank, and if yes, who should he replace on my team?

Current team:
5* KM 2/35 unduped
5* ST 1/25 unduped
4* Sparky 4/40 duped
4* Ghost 4/40 unduped
4* Gamora 3/30 unduped

Thanks!
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