If a standard car stereo has 8 ohm speakers attached, should it be okay to install 4 ohm speakers, or should I get an external amp just in case. I would rather avoid installing an amp, but if required I will.

Thanks for any help

By using 4 ohm speakers in place of 8 ohm speakers, whatever is driving them will have to supply twice the current... If whatever is driving them is not 4 ohm stable, it will likely shutdown or cook itself.

You can put a high wattage 4 ohm resistor in series with your 4 ohm speakers, if you REALLY don't want an external amp, but the resistor will be absorbing half your power.