It's important to use your audio components within their rated parameters. Amplifiers have minimum ohm ratings and you should not connect speakers to them which have a lower rating. But there is a way to use lower impedance speakers with an amp that has a higher impedance rating. The method is called serial wiring and you can safely hook up 4-ohm speakers to an 8-ohm amp by using this method.
Turn off the power to your amplifier while wiring the speakers. In this case, shutting off the ignition and accessory power is sufficient.
Remove one-half-inch of insulation from both ends of three single strands of speaker wire using wire strippers. One wire needs to be long enough to run from the speaker terminals of your amplifier to the terminals of the first speaker in your system, the second wire must be long enough to reach from the first speaker to the second and the third wire long enough to reach from the second speaker back to the amplifier.
Loosen the positive and negative output terminals of your amplifier's speaker terminals with a screwdriver. Loosen the terminals of your speakers by twisting the speaker terminal knobs to the left to reveal a small opening for the speaker wire.
Connect the first wire between the amplifier's positive (+) output and the first speaker's positive (+) terminal. Tighten the terminals to hold it in place.
Connect the second wire between the first speaker's negative (-) output and the second speaker's positive (+) terminal. Tighten the terminals to hold it in place.
Connect the third wire between the second speaker's negative (-) terminal and the amplifier's negative (-) terminal. Tighten the terminals to hold it in place.
Turn your system on; you can now use your amplifier with the two speakers connected.