Flight time would not have been an issue, you could have gone. The FAA example has a crew divert to an alternate, repo back to the planned destination and still depart for the last leg.
http://www.faa.gov/about/office_org/...2009/Banks.pdf
One thing they point out is you still have to be able to look back and find your rest. If you get 30 minutes after block in, for your 5th leg you'd have to be able to say at 19:30 that at 21:45 you could still look back and find 10 hours of rest.