Vaughan supports it on the basis the average test lasts 319 overs so 90 a day should normally see 4 days, however all you need to say is the ashes 2-2 and a drawn 4th test or rain affected which would be sorted day 5.
Perhaps a suggestion, 4 day tests that have to see 90 overs a day, if 90 overs a day aren't played it goes into a 5th day?