For slotting with a 4mm HSS 1 flute cutter at 4mm DOC I am seeing 18881rpm and 2333mm/min on my calculator. You are right 0.238mm chip loading.

Carbide is 24000rpm and 2471mm/min with the other parameters the same

That's with a 15mm stick out.

That's for a finishing pass - roughing would be way higher - carbide 24,000rpm and 4745mm/min