Every speaker has a particular impedance, which is measured in ohms. This value shows how difficult it is to power a speaker. The lower the speaker’s impedance, the more efficiently it passes a signal (namely music) through it. The majority of speakers are 4,6, and 8-ohm models. If you use a cheap receiver, you may face problems with using low-impedance speakers.
Some people mistakenly think that 8-ohm speakers always have this impedance, but it can vary. The main factor that affects impedance is the frequency at which a speaker is playing at a particular moment. The truth is that 8 ohms speakers can be 7, 29, and even 44 ohms. It is directly dependent on the frequency.
If you wonder whether you could hook up 8-ohm speakers to a 4-ohm amplifier, you need to clarify how the latter works. Nowadays, solid-state amplifiers are the most popular options. They resemble voltage sources with impressively low output impedances. If a load is higher than the minimum specified, an amplifier will work as needed.
If you add 8 ohms speakers to a 4-ohm audio system, there won’t appear any problems, e.g., reduced frequency response, increased distortion, etc. Just remember that the load caused by a speaker to an amplifier is largely affected by frequencies. So, if you get a decent amplifier that is capable of maintaining a 4-ohm load, you can safely pair it with 8 ohms speakers.