Transistor amps adjust automatically to the impedance of various speakers.
Here is an analogy that might help. Your typical 120 volt wall outlet in a home is capable of delivering up to 15 amps of power to any device you plug in. That's about 1800 watts of power.
However, when you plug in a clock radio that only needs 15 watts to run, that is all the wall socket delivers - 15 watts. If you unplug the clock and then plug in a toaster that needs 1000 watts, the wall socket sends 1000 watts to the toaster. There is nothing for you to "adjust" on the wall socket in either case.
This analogy isn't perfect (wall sockets don't have volume controls) but it gives you an idea. A lower impedance speaker (4 ohms) will attempt to draw twice as much current from a transistor amp as an 8 ohm speaker. This translates into the 4 ohm speaker receiving more watts.
Some power amps are not comfortable with low impedance loads so you shouldn't hook up a speaker to an amp if the speaker has a lower impedance than what the power amp's owner's manual indicates is appropriate.
Tube amps are generally a different story. The output tubes hook up to the speakers through an output transformer. These transformers often have one set of connections for 4 ohm speakers and a different set for 8 ohm speakers. Use the connections that provide the closest match for your particular speakers.
Here is an analogy that might help. Your typical 120 volt wall outlet in a home is capable of delivering up to 15 amps of power to any device you plug in. That's about 1800 watts of power.
However, when you plug in a clock radio that only needs 15 watts to run, that is all the wall socket delivers - 15 watts. If you unplug the clock and then plug in a toaster that needs 1000 watts, the wall socket sends 1000 watts to the toaster. There is nothing for you to "adjust" on the wall socket in either case.
This analogy isn't perfect (wall sockets don't have volume controls) but it gives you an idea. A lower impedance speaker (4 ohms) will attempt to draw twice as much current from a transistor amp as an 8 ohm speaker. This translates into the 4 ohm speaker receiving more watts.
Some power amps are not comfortable with low impedance loads so you shouldn't hook up a speaker to an amp if the speaker has a lower impedance than what the power amp's owner's manual indicates is appropriate.
Tube amps are generally a different story. The output tubes hook up to the speakers through an output transformer. These transformers often have one set of connections for 4 ohm speakers and a different set for 8 ohm speakers. Use the connections that provide the closest match for your particular speakers.