The fourth ATR is used on schedules but is often kept as a 'spare' & due to frequent tech issues gets well utilised. However the 68 seat DB not ideal for covering full loads on BO & ON as they are 72 seat & none of the ATR's are useful in covering full loads when the Embraer goes tech - this under capacity cover often leaves pax stranded.
BOHEuropean - I had the info from a good source who is usually spot on with his information.