I always sift & then measure, but I don't know if that is correct. I just thought I'd let you know that my items have come out okay, so maybe there's not that much of a difference in quantity & it's more of a quality issue. In fact, I normally start with the same measurement as I end-up with.
Here's what I was taught back in the days when they called it Home Economics (now do you know how old I am?? LOL):
Sifted flour/sugar -- sift first, then measure.
flour/sugar, sifted -- measure first, then sift.
Theresa 
I believe that if the recipe reads for 1 cup of powered sugar :
1 cup Sifted powered sugar - this means to sift before measuring
1 cup powered sugar, sifted - this means to sift after you measure it.
I hope this makes since, if not just let me know.
You can remove some flour from the cup if you want to, but if you are in humid conditions, you should sift anyway, to eliminate lumps. Do not break the lumps down through the sifter, throw them away. It means they have caked together, and lump up again.
Theresa
I believe that if the recipe reads for 1 cup of powered sugar :
1 cup Sifted powered sugar - this means to sift before measuring
1 cup powered sugar, sifted - this means to sift after you measure it.
I hope this makes since, if not just let me know.
Monica is correct! It's how the recipe reads that matters.
It's the same with any ingredient. If the method of preparation is listed before the item, you measure after preparing it (ie: 1/2 cup chopped nuts). If the method of preparation is listed after the item, you measure it before preparing (ie: 1/2 cup nuts, chopped)
I had Home Ec (& thats what it was called) when I was in middle school & I am 35. When did they rename it? Whats it called now?
Yes that does make sense. It says 4 c. sifted CF.
So I need to sift than measure.
Correct.
This is interesting because in a Taste of Home Cooking School - they taught us to take a wisk and 'fluff' the flour (they weren't talking about powdered sugar right then) and then use a scoop to gently pour it into another measuring cup.
I do that with flour, only, and then I sift it with other ingredients - or by itself.
But with powered sugar - most of it is already very light and fluffy so I just measure it - and then sift to blend with other ingredients.
The big 'no no' we were told was to just scoop flour out of the canister - because it had a tendency to pack down - making your recipe dry and tough.
I'm not sure that's all true - but that's what we learned.
This whole thread is why I don't measure, I use a scale. Flour can get packed and I'm sure powdered sugar does also, so measuring is just not accurate.
I never used to have consistent results, sometimes the icing was stiff sometimes the icing was thin, now I have consistent results.
And not having to wash measuring cups really makes a difference in clean up time.
I got the digital scale at walmart for around $30
WOW 
I just sifted 6 cups of Domino's CF. You would not belive the lumps in it. These bags were purchased 3 hours ago! No humidity in my house, AC has been on for over 40 hours. I estimate about 2/3 cup per bag of lumps the size of the point of a pencil.
NEW QUESTION: Does anyone know of finer CF than 10X?
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