Let’s not overcomplicate things. If ND is in the top 4, they are in the top 4. They get a first round bye like any one else in the top 4.

If the quarter-final sites are the Rose, Sugar, Orange, and Fiesta then the other 3 in the top 4 get their traditional tie ins and ND gets the bowl that is left over.

Alabama: Sugar
Clemson Orange
Ohio St: Rose
ND: Fiesta

One of the keys to a successful playoff is an easily understandable selection and placement process. the top 4 seeds should get to host their quarter-final at their traditional bowl site. The only time this would be a problem is if:

The Big Ten and PAC 12 both have a team in the top 4. Solution: Rose Bowl goes to the higher ranked school. The lower ranked school goes to whichever NY4 bowl doesn’t have a host school in the top 4.

A conference produces 2 or more teams in the top 4. Solution: The Conference Champion gets the traditional tie in. The other(s) pick from the remaining NY4 bowls.