Are Monarch sticking with four aircraft do we know for Summer '18?
Beyond April there's no flights to Lanzarote, Ovda, Porto, Rome, Stockholm or Tel Aviv plus a couple of reduced frequencies from this summer. Not sure whether these have all been cancelled? Their schedule though is all over the place at the moment so there's certainly some rejigging to do.
I make it only 43 weekly departures in August 2018 at present. Unless there's a good number of new routes to be announced it looks to me at this stage like, after a bit of rejigging, that only three aircraft maybe required.