I am running four rolls of paper, splitting it in half at the slitter section prior to cutting it. At the pile, the width of each pile varies noticeably. Why?

Are you taking an edge trim on each side of the rolls? Widths vary to within 0.015” (0.38 mm) from roll to roll. When put on a roll stand, incoming webs can wander as much as 0.125” (0.17 mm), particularly from the furthest roll stand. To ensure width accuracy on multiple pile work is maintained, you need to take edge trim or have individual web guidance.