I scoop with a half quart bowl as I weigh, and it doesn't hold a full pound. 4-5oz per cup sounds about right.
But, you should really just get a scale. You can't do this hobby without a good scale (hops, grains).
Do you have a bathroom scale? If so, weigh yourself holding a bucket. Then fill the bucket and keep weighing yourself until you've gained 4lbs (or however much grain it is you need to weigh).