W09 timetable shows early morning departures to LGW, LPL, MAN and BHX, so clearly the plan, right now is to have 4 Q400's at Ronaldsway. However it's 3 months away so anything could happen.
Second MAN rotation will be operated from MAN, as now, so difficult to see what the 4th aircraft will do between 9am and 5pm. Hot spare maybe? - IOM is a good central location to reach several key BE bases in well under an hour?