how does current work in an amplifier?

The KISS answer to your second question is YES. An amplifier rated for a continuous 4 ohm load should handle 4 ohm nominal speakers better than an amplifier that is only rated for a continuous 6 ohm load. You're probably safe to make comparisons within a brand, but I don't think advertised specs really mean much when comparing different brands.

A formula that might help is the following:

Current in Amps SQUARED = Power in Watts DIVIDED by Resistance in Ohms

For comaprison:
100 watts into an 8 ohm speaker requires 3.54 Amps
100 watts into a 4 ohm speaker requires 5.00 Amps

Not sure if this helps at all, but...

Dsper - the owner at the store I deal with is constantly telling me to ignor specs and just listen. He's given me some extrememe examples of equipment that he's heard where the actual sound was the opposite of what was expected. It was something crazy like 9 wpc and a THD in double digits that he said was amazing.

He also noted that there isn't really any true regulations going on to verify specs. Many of the specs are taken at the one and only ideal frequency. Think Ford commercials.

Specs are like statistics.

The only statistics you can trust are those you fabricate yourself.