For low winds in an Hawt configuration (horizontal) I'm of the opinion that fewer is better (perhaps stick to three for simplicity). I think the trick is to increse the blade width (cord) over that recommended for a regular machine. You don't see airplanes with 5 wings anymore because the parasitic drag from non productive and productive portions of each wing is killer.
Seeing as how removing the part of a wing that is producing lift to reduce drag is extreamely counterproductive you can (and they have) reduced the number of nonproductive portions like the tips and the wing/body connections etc. Of course a blade profile that has a really good lift/drag ratio helps alot too.
When searching around the web for good blade profiles remember that the wind speeds across you turbine blades would send almost all airplanes into a nosedive be carefull to locate profiles that are designed for low windspeed (low RE -reynolds number-) applications. Generally the blades will be thicker and shorter in relation to thier width. Wide, thick and stubby wings are called high aspect ratio wings and are generally used for heavy lifters and are considered innefficient but I think people make a mistake when they apply the low aspect ratio theory to a low wind speed wind turbine.
In short I say go with three blades that are thicker, wider and shorter in relation to thuer width than you'd normally see. You may loose some high wind efficiency but I'm confident you'll pick up big time on the low end.
Mike
P.S. The only airplane designer I can think of who bucks the fewer is better concept is Burt Ratan and I'll admit it's hard to argue with his genius but he also does alot of carefull work to reduce parasitic drag from the wingtips and wing/fuselage areas.
