You can use 4ohm speakers on a 2ohm amplifier. It is ok to have higher impedance load, than previous. They dont make 1ohm speakers so you are ok.
If you added another 4ohm speaker in parellel, than the total impedance would be 2 ohms. This would allow you to add in some remote tweeters, say, in the headliner(of course need built in FX crossover) that are also 4-8 ohms without worry of overloading the final amplifier. Most tweeters are 8-16ohm.
Matching of the amplifier impedance to the speaker impedance is the most efficient power transfer. The load(speaker) matched impedance allows more current to flow. Higher impedance restricts the current flow. Not in a bad way though. Sometimes a high quality 4ohm speaker is much better to use than a low quality 2ohm speaker.
Back when I would repair stereo systems, kids would get all hopped up on the sales pitch of having 2ohm systems. Electronically speaking the THD (total harmonic distortion) is what is most important as that is really rating the efficiency and quality of the amplifier. A 2ohm amplifier allows you to hook up one 2ohm speaker only, or two 4 ohm speakers in parallel.
What do you think delivers the best sound experience? one placed speaker, or two creatively placed speakers? Think about your 5.1 home surround system. 5(5.1) creatively placed speakers delivers a better sound experience than a 2.1 (3 speaker) system. And requires less power from the amplifier to deliver that sound experience.