how much power

Tls49,
If the amp is rated for 6Ohms minimum, than it will not have enough current for 4 Ohm speaker. Not only the output power is jeopardised because of mismatched impedance and lack of current, but also the remaining power is jeopardised by heat.
If OP gets half of rated power to the speaker, consider that as a best case scenario where the worst is fried amp even on moderate 75dB volumes.

Conserning Onemug's statement in general it's true that 50W and 100W will have a slight difference in SPL but in this particular case it's a substantial difference.

So Roxito,
You can have amp as low as 50Wpc, but it should be rated stable for 4Ohm load or preferably lower.

Rok2id, the amount of specified current isn't important. A stability to the certain load is. An amplifier that delivers 60W onto 4Ohms presumed to have sufficient current to drive 4Ohm speakers.

Magfan, Rok2id,
45 amps is peak current that has nothing to do with operating continuous collector voltage of amplifier.
Continuous current would probably be in the range of 1...2 amps.
What is necessary for amplifier to deliver the same power onto the 4Ohm load vs. 8Ohms is to double the current and not on peak or transient bases but on continous.
Not all amps are capable to weld but there are some.

Lacee,
I used to drive Chevy Suburban, but now getting around with Toyota Camry. Camry in general is OK, however it will less-likely pull 25' Donzi on trailer.

While I might concur to your case Lacee,
I can't imagine swapping to 2...5W amp driving my Aerial 10T. Now they're driven with approx 700W of power in the medium size room with listening distance approx 5.5' and the same between speakers. I'm fairly satisfied with sound, but have to put some effort with acoustic treatments and probably switching to electronic crossover. The built-in one somewhat makes me think that it's not properly balanced.

Good suggestion about Kreel:-)! I'd probably pick KSA250. It drives anything by any means!

Charles1dad,
By all means low-wattage amp can't reproduce real volumes of let's say acoustic upright bass even in the small room. High-efficiency low-wattage friendly speakers can't have sufficient bottom-end performance and lower efficiency full range speakers can't be driven by a few watts of power.