Most amplifiers support 4 ohm speakers just fine. If you want to be sure, find the specs for your amp. There are no switches to set.
The lower the ohms, the more power the speakers will pull from the amplifier at any given time.
Ohms (a measure of "impedance") are a pretty approximate thing. An 8-ohm speaker can act like a 4-ohm or even a 2-ohm load depending on the music being played, and will vary during a song. So amplifiers are for the most part used to dealing with varying impedance.
The lower the ohms, the more power the speakers will pull from the amplifier at any given time.
Ohms (a measure of "impedance") are a pretty approximate thing. An 8-ohm speaker can act like a 4-ohm or even a 2-ohm load depending on the music being played, and will vary during a song. So amplifiers are for the most part used to dealing with varying impedance.