An ohm is a measure of electrical impedance. Ohm ratings are used to determine the highest efficiency of speakers and amplifiers, and should be matched carefully.
An amplifier that is rated at 100 watts at 8 ohms should always be matched with an 8 ohm speaker equal to or greater wattage. When the speaker ohms go down, the amplifier provides more power, which can damage both the speaker and the amplifier. When ohms go up, the amplifiers produce less power, which is good for the speakers, but less efficient for the amplifier.
Some guitar amps have built-in ohm selectors to allow the player to use a variety of cabinets without amplifier damage. Although this feature protects the amplifier, the power output water still increases at lower ohm settings and decreases at higher ohm settings. The wattage of the speaker should be carefully considered to avoid overcurrent.
Combo amps, which are integrated amplifier and speaker devices, use matching speakers and amp components. If an external speaker is connected, always use the correct ohm rating, which is printed near the external speaker socket.
There is no sound or volume difference between 4 or 8 ohm guitar amplifiers when matched with the correct speakers. Choosing one or the other is a matter of amplifier, and the type of external speakers to be used, if applicable. Resist the temptation to experiment without knowledge. Amplifiers and speakers are sensitive, expensive parts of electronic equipment, and training in proper use will help you get the most effective and safe results. The choice between 8 ohm and 4 ohm guitar amplifiers depends on the intended use. Guitar amplifiers are rated in the output water at a certain ohmic number and must be matched with speakers with comparable ratings.