．。
4. Heat sesame oil in a wok and melt butter in it.
Literal
4. Wok [in-に] sesame-oil [object-を] heat butter [object-を] melt.
A recipe step in the instructional plain-form register. Two actions chained with the written connective form 熱して — 'heat the sesame oil and [then] melt the butter.' 中華鍋 ('Chinese pan' = wok) is a cooking-specific vocabulary item — the full Sino-Japanese compound for this pan. The prepositional sequence here is worth noticing: the wok is the locative with に (where the oil goes), and then within that same wok, the butter melts — no explicit に on the butter, because the frame is already set by the heating clause.