If a car has a standard 4 ohm amplifier receiver, then 8 ohm speakers can be safely used. The amplifier plays a role in defining how safe it is to use different types of speakers, as lower ohm amplifiers should not be paired with lower ohm speakers.
Speaker impedance is generally static and does not stay the same number. Depending on the amount of wattage it sends, a 6 ohm speaker can reach impedance numbers as high as 50 ohms, to as low as 4 depending on its setting.
Pairing lower rated speakers, such as 4 ohms, with low power rated receivers is where a problem can arise. Speakers rated at 4 ohms use more power than those rated at 6. If paired with low power receivers, a unit can burn out.