Have you heard of the -4B/C? The -4B was a field-modified -4 equiped with four 20mm cannons (2 per wing) and the -4C had factory installed cannons.
Yes, I have.
The F4U-4B was the designation for F4U-4s earmarked for export to the Royal Navy, however no such aircraft were actually produced so the designation was rescinded. After the war it was recycled to designate F4U-4s in American service armed with 20mm cannon, as opposed to the .50cal. Key word: AFTER the war.
The -4B designation had NOTHING to do with a field mod of anything. F4U-4Bs were factory-armed with 20mm.