Yes - they might be able to find one aircraft for next summer but four?? Where from, crews? etc, etc...
It's now the middle of December - shouldn't they have flights on sale?? etc, etc....
Aircraft availability wise, according to Jethro Jet2 have two unfilled registrations for a 300 and a 800 variant of the 737. Also how safe is Jet2s Blackpool base?
The airline is announcing routes regularly at the moment so selling the seats may not be as big a problem as thought but time is tight though.