I'll have to ask one of my contacts in Jet2 Engineering next time I see him. He's mentioned -400s before but not for a few months so things may have changed in this fickle business.
I s'pose it would make sense to go for the -400. I expect there's a reasonable number available and if they've got a bit more range than the existing fleet they'd be okay for the likes of Larnaca, Paphos and the Canaries whilst hopefully carrying more punters.
The -700 and -800 would probably stretch the purse strings of Dart Group a little too far at the moment. Still, whatever they get, the more the merrier.