If the manufacturer recommends 4 ohms, they most likely included the option for 8 ohm operation because most surround receivers can't handle a 4 ohm load. Since you're using an amp that can handle 4 ohm loads just fine, I'd set the speakers to the manufacturer's recommended 4 ohm setting and go with it.

Having an amp rated for more power than a speaker is "rated for" (in quotes because power ratings on speakers are somewhat questionable numbers) won't hurt anything unless you plan to crank the volume up to a level that will cause your ears to bleed. It's actually better to have some extra power than not enough, because having too small an amp may lead the amp to clip. Clipping is a sure-fire way to damage a driver.