Hi Emma,
I'm doing the four days as Tuesday, Thursday, Saturday and Sunday, so would probably try to do the long run either Tuesday (preferably, so you have more time to recover before the next one at the next weekend but a bit late now!) or Thursday (probably too close to the next one), as its the long run, with the time on your feet what you need to get used to I think.
Mind you, its still early in the schedule for you, so if you've missed it, well, accept it, cos you don't want to go mad trying to make it up and hurt yourself.
Having said that, there are loads of people on the forums that are more experienced than me - lets hope someone offers us both some advice!
Good luck with Plymouth - Berko was hard, which is why I thought I ought to approach Keswick sensibly!