I'm not sure the tie breaker rules on points spread, but there is usually a 21 point win margin max. If that's the case, in the event of a 3 way tie for 4th between Marshall, Nac and MtP/PT winner:
Nac enters tonight (-21) after getting blown out by both Mt P and PT.
MtP/PT winner would be even-(0) after getting blown out by Marshall and blowing out Nac
Marshall enters tonight at (+21) from beating both Mt P and PT by alot.
That would mean Nac needs to beat Marshall by at least 21 points tonight to have a chance to get in. In the event of a record, head to head, and a points differential tie, it usually goes to coin flip I believe.