I've asked this question here some times, and have noticed others have as well. So these lenses are undoubtly popular, but many newbies like myself always ask the question, will the difference from f/2.8 to f/4 make a difference when shooting outdoors?
From playing around with my Sigma for one day, I just have to say _YES IT DOES_. In sunlight or other bright light, probably not. But today when outside, The sky acted like this giant softbox (which it often do in Norway in autumn/winter). Don't mistake, it wasn't dark. It was bright indeed, but no direct sunlight. I had a hard time handholding 200mm @ f/2.8 today. At ISO-100 my shutter-speeds where as low as 60-90. I tried stopping it down to f/4 to see what difference it would make, and it was huge! Totally impossible to handhold (not a slightest chance @ 200mm)
Well that was just my experience from 20min of shooting in my garden today. I know for sure, that if this had been a f/4, I wouldn't have gotten any pics at all.
So consider this before buying, outdoor light doesn't neccessarely have to be bright, even though at 12:00pm. Just my two cents.