It's all a bit complicated. The impedence shown on your speakers is the
nominal impedence. This is what the manufacturer states BUT the impedence of a speaker changes significantly depending on what frequency is being played. The upmarket B&W models for example are rated at 8ohms but can swing down to below 2ohms at low frequencies! Conversely, my Mission 771e's are also rated at 8ohms but don't drop below 6ohms at any point.
The numbers quoted by manufacturers are generally pretty useless. The ideal amplifier will double it's power output as the load impedence halves (ie. Provide 200W into an 8ohm load and 400W into a 4ohm load). This is very difficult to do and only the very high end models get anywhere close to this.
What you have to remember is that the
voltage supplied to a speaker is dependent on the volume so at a constant volume, twice the
current must be supplied to the speaker to double the power provided. Current supply is where a lot of amplifiers fall down, they may be fine for 8ohm loads but they certainly won't be able to provide twice the power into a 4ohm rated speaker.
There are far too many variations but remember that you need a better specified amplifier to drive speakers that have a low impedence.
Your manufacturer, for example, states an 80W output into a single 4ohm load. If it were an ideal amplifier it would only provide 40W into an 8ohm load. This is also for a single speaker, if you're using more than one at a time you can expect these values to drop further.
Hope that helps a bit but as I said at the start, it's a complicated area and most manufacturers are not to be trusted in it!