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:

2 x 300W at 4 Ohms 
2 x 300W 4 Ohm 
600W 
100% 
2 x 300W at 4 Ohms 
2 x 300W 8 Ohm 
420W 
85% 


... 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.


Running more than 2 speakers from a power amplifier 
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. Working on the example at the top, if
your speakers are 250 watts RMS each at 8 Ohms, then connecting two
in parallel will change the load to 500 watts at 4 Ohms per side, so
you have to find a power amplifier to match that.
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.
Still confused? Email
enquiries@whatpa.co.uk and we'll try to help! 
