The amp must be capable of driving 4 ohms. The amp is generally designed to specs such as 100 w/ch into 8 ohms both channels driven. It also could have specs for 4 ohms and sometimes even 2 ohms.
With R constant, you can see that P = I^2*R and I can limit the power output. P = (V^2)/R and in this case V^2 can limit the power. In reality, the smaller of the 2 limits power. The speaker doesn't have a constant Z of 8 or 4 ohms or whatever, so they may be designed for 6 ohms, but only for short periods.
One advantage in a car is because of the 12 V power supply with a huge current available. In bridge mode, you can effectively operate from a + and - 12 V supply, thus lowering the speaker Z has an advantage.