From what I understand, it's 4 days off between "work blocks", not "trips". So, you may have a 4 day trip, backed up to a 2 day trip (or any conceivable combination of trips), to complete your "work block". Seems like that would encourage the company to fill up six full days of flying with any crazy combination of trips, just to maximize your "work block" efficiency. I could be very wrong. Maybe a Spirit guy can chime in how this work rule plays out in practice, with regards to actual schedule creation.