u'r prett much screwed since u already saved in 5.0... otherwise i would say to revert to an older version of maya, save your file as .ma (maya ascii) then edit it with ur favorite text editor... (probably something microsoft:o ), and in the header somewhere find where it says: opens with, or something in that nature, and change the current version back to the version u want to open with.. in this case 4.0
hope this helped