4 Ohms into 8 Ohms does go ...

Most stereo power amps operate at 4 Ohms per channel. If you connect 4 Ohm speakers to each channel then you will get the full wattage of the power amp.

However, if you connect 8 Ohm speakers to each channel, the wattage of the power amp will be reduced by around 30%. In terms of perceived loudness, the amplifier will seem about 15% quieter. For example:
 

... but 8 Ohms into 4 Ohms NEVER GOES!

Never, ever connect a PA amplifier into speakers with a lower Ohm rating - this will result in either the amp or speakers (or both) being damaged.
 

Doubling up speakers

If you double up speakers connected to each channel of your power amp, then how you connect them together will affect the combined number of Ohms.

If two speakers (of the same rating) are connected in parallel then their Ohm ratings are halved. If they are connected in series, then their Ohm ratings are summed.
 

How do I connect 2 speaker cabinets in Parallel?

Most passive speaker cabinets have 2 jack plugs at the back. If you connect the amplifier to speaker 1 via the first jack and then connect speaker 1 to speaker 2 via a jack to jack lead, then the 2 speakers will be connected in parallel. If each speaker is rated at 8 Ohms then the combined rating is now 4 Ohms.

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