What size tire on a 15x4 inch rim ?

TireRack.com provides charts for the tires they sell that specify the minimum rim width for each tire size.

A 165/80 is a tiny tire for a heavy car — that would have been the stock size on my 2000lb MGB in 1966. An A-body is putting at least 300lbs extra on each corner. I suppose if it's a race car it doesn't matter.