4 ohm vs 8 ohm

If you measure the resistance of the voice coil of a 4 ohm speaker, it is not 4 ohms. That is because with speakers, the term 4 or 8 is the IMPEDANCE of the voice coil, not the resistance. To complicate matters, impedance is also quoted in ohms. The actual resistance [ impedance ] varies with frequency; impedance is the resistance to alternating current [ AC ], resistance is the resistance to direct current [ DC ]. The quoted impedance of a loud speaker is the nominal impedance.
What I said in post #4 is correct. In electrical theory there are 'theorems & laws', such as Thevenim's. Another one is the 'Maximum power transfer theorem', which states that the output of a device will transfer maximum power to the load when the input of the load has the same value [ or load ] as that of the output device. If the car radio has an output impedance of 8 ohms, max volume & clarity will be obtained with an 8 ohm speaker. Using a 4 or 16 ohm speaker will result in max volume not being as loud & some possible distortion...but it will work ok.