A.T. Robertson Commentary 1 Corinthians 4:21

"What will ye? shall I come unto you with a rod, or in love and a spirit of gentleness?" — 1 Corinthians 4:21 (ASV)

With a rod (εν ραβδω). The so-called instrumental use of εν like the Hebrew (1 Samuel 17:43). The shepherd leaned on his rod, staff, walking stick. The paedagogue had his rod also.

Shall I come? (ελθω; ). Deliberative subjunctive. Paul gives them the choice. They can have him as their spiritual father or as their paedagogue with a rod.