Right Speakers?

I used to use 8 ohm speakers for 4 ohm radios all the time. Never experienced any issues.
Most car radios of the 60s and newer have solid state amplifiers. These act as voltage sources, with very low impedance output. As long as the load exceeds the minimum specification, then the amplifier should perform without issues. Using an 8 ohm speaker should not result in any loss of quality, distortion, reduced frequency response or affect the system itself. Actually, the load presented by a speaker to an amplifier varies with frequency anyway so the recommended speaker is the best middle ground selected by the radio manufacturer.