I love this dip. I agree that the spinach should be added after blending everything because otherwise the dip will be a thick dressing consistency (but not runny; that means you overprocessed it). I used this dip for carrots, mini bell peppers, and crackers with no problem. Doesn't need salt.
4 cups of spinach is based on volume, not weight. In the end, boiling the spinach reduces the size AND volume of the spinach significantly.