With solid state amplifiers you can safely add speakers with a higher impedance (e.g., 8 ohms or 16 ohms instead of 4 ohms), but the other way around has risks.

When you connect a 4 ohm speaker to an amplifier nominally rated with an 8 ohm load, the speaker will draw twice as much current (at full volume) as the amplifier is designed for.

Depending on the amplifier, this might have little or catastrophic effects. My amp has a thermal cutout, and connecting a 4 ohm speaker would cause the amp to cutout at high volumes. When it cools down it works again. At low volumes it would not matter, as the amp will not be overloaded. Even with a 4 ohm load, the amp will only be overloaded at the extreme of its output, because that last overload current (from rated output to double output)only gives you an extra 3dB of volume, which you can barely hear. If you don't play music loud you won't harm the amplifier.

Of course, if you overload any amplifier (so it is distorting) you risk damage to the both the amplifier and speakers.

The danger is that an amplifier without thermal protection could be burnt out if you run full volume into 4 ohm speakers. So, you are taking a risk.

You can connect 4 ohm speakers in series to give you 8 ohms, but make sure that they are in phase. A number of manufacturers (e.g., Bose, Wharfedale) use multiple speakers in series/parallel arrangements to give the correct impedance.

Regards
Chris