In many cases, 4-ohm speakers sound a bit more exciting and punchy than 8s, whereas 8s are more refined.
This is a generalisation, and I don't even know if it's true, but I've definitely heard it said...
For a given wattage, two different speakers -of the same sensitivity-, one 8ohm and one 4ohm, should produce the same level of volume, in theory.
The four ohm speaker will draw twice the current of the 8ohm, but at only half the voltage across the terminals, as Power=current x volts, if either goes up, the other goes down to compensate.
It is not really an issue- a good speaker is a good speaker regardless of its impedance, and there is no specific downside to 4ohm speakers. And as long as you don't want rock concert volume levels out of them for long periods, they shouldn't trouble any half-decent amp.
As for your second question, I don't do AV so I haven't a clue!