Use a different amp, or run another 4 ohm speaker in series with your speaker.
IMO most amps should be made for 4 ohms, but most home amps are 8 ohms min
What I do for my setup: (My receiver is 100W/ch @ 8ohm)
My speakers are 2 boxed car speakers with two 8" woofers + tweeters, 4 ohm in each speaker.
I wire the two 4 ohm speakers in each box in series and
in phase so I get an effective 8 ohm speaker for each channel.
That's the best way to do it without having to waste power in a resistor, or overload the amp. Just connect speakers in series.