[curt.clifton]

In lieu of built in support, here's how I'd handle this now:

I'd set the project due date to "now", put no due dates on any of the items, then change the due date of the project to +4w after mailing the receipts.

It's not as automatic as having items infer due dates based on the due dates of their successors, but it gets the job done in the meantime.

Hope that helps.