When an amplifier is connected to a speaker, the impedance of the speaker load affects the amount of current that flows through the amplifier's output stage. The impedance of a speaker is measured in ohms, and the lower the impedance, the more current will flow.
When an amplifier is connected to a speaker load that is too low, it can cause the amplifier to overheat and fail. This is because the amplifier is trying to push too much current through the speakers, which creates excessive heat.
In the case of a 4-ohm guitar amplifier connected to 16-ohm speakers, the impedance of the speakers is too high for the amplifier. This means that the amplifier will not be able to push enough current through the speakers, which can result in a weak and distorted sound.
It is important to always match the impedance of the amplifier to the impedance of the speakers. This will ensure that the amplifier and speakers operate safely and efficiently.