I'm struggling with setting up Units of Measure for some items.

Let me give an example:
Bubble Wrap.  We use bubble wrap on some shipments.  It's on a Roll and you tear off the amount you want.  I sell it in lengths as part of a Kit/Assembly. I want to track inventory as rolls.  We purchase as bundles which are 4 rolls.  So the Vendor part number is for a bundle.  The pricing is per bundle.

So already I'm confused.  How do I set this up?  If I don't use Units of Measure and stock as bundles I will always need to multiply the On Hand amount by the amount in a bundle which could change some day.  If I don't use Units of Measure and stock as rolls I will need to divide the amount I want by the bundle count and worry about rounding.