Most of them are 4 ohm; in fact, 4 ohm speakers are pretty standard in DC-powered sound systems. The reason for this is because DC systems are usually being fed a low voltage, in the neighborhood of 12 volts, and you need bigger/bulkier amplifiers, and bigger currents (amperes), which need bigger power and ground wires for a given level of amplification than you do for a home system running on 120 volts. That's why a 1000 watt home amplifier can have a typical 16 or 18 AWG power cord while a 1000 watt car amplifier would have at least 4 AWG power and ground wires.
In any event, using 4 ohm speakers rather than the traditional 8 ohm speakers that are usually used in home audio, helps to offset this, because they will pull twice the power from a given amplifier design and given input voltage than 8 ohm speakers will (assuming a typical amplifier design).
In any event, it is perfectly okay to use 8 ohm speakers in a system designed with 4 ohm speakers in mind, but you'll only get half the power from your amp. It is not recommended to use e.g., 2 ohm speakers in a system designed for 4 ohm speakers, because you are pulling twice the power through it compared to 4 ohm speakers (again, assuming a typical amplifer design).
So going with the higher impedance speakers gives your amplifier an easier life, at the expense of power delivered to your speakers. Going with lower impedance speakers gives your amplifier a harder life with the benefit of more power delivered to your speakers.