4 ohm is correct for an aftermarket radio.
8 ohm is minimum for an original Delco radio, but they recommend 10 ohm.
4Ω speakers will ask for too much current from the amplifier circuit, and burn out the bias resistor and amplifying transistor (ask me how i know, and ask me how to fix it if it ever happens to you :) ). You will need to add a resistor in series, not parallel, to use that speaker in an original radio system. It won't affect your sound, the speaker reacts to voltage not current, and that won't change.
Thank you for the correction ( especially when you are right )
Trying to keep this fellow out of trouble but not working well this time ——Opps !
Looks like you just could have saved him a possible radio issue
He does not have the correct speaker
I knew not having the right resistance could damage the radio