I'm pretty sure it's nine cups. All the recipes I've used that called for one pound of PS said 4 cups.
I was making this icing usine the 1/2C as the 4oz, until last night. I decided to get a $7 kitchen scale at Wal Mart, and weigh it. When the 4oz is weighed, it is much more than 1/4C. Next time I make the cookies, I'm going to weigh the flour if I can figure out somewhere online how much 1C of flour should weigh. I have a feeling I'm getting too much flour when I measure. I would suggest investing in a cheap scale for measuring the 4oz of sugar!
OOPS, I was a little off on my math, 
sorry. Thank you for the correction on it, now I can get it straight in my head too.
here is a link to a page that I find helpful with conversions
http://allrecipes.com/HowTo/Commonly-Used-Measurements--Equivalents/Detail.aspx
HTH
Here's a link to a chart Crimsicle put on CC. It's very helpful.
http://www.cakecentral.com/cake-photo_25279.html
Since getting the cake bible, i weigh everything. It's much more simple and accurate. all you have to do is keep adding ingredients to the same bowl. I hardly every use my dry measuring cups anymore.
Get a scale that can give ounces and grams. It's great.
Enjoy.
kelleygirl
When I make this recipe, I use 9 cups of powdered sugar. Normally, 4 cups of powdered sugar is one pound so 4 ounces is 1/4 of a pound which would be one cup. It works for me with the 9 cups anyway LOL
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