earlier I peeled 4 potatoes

[RattleAndRoll]

I didn't use them in the end tonight.
Tomorrow we're not home for dinner. Will they last til Saturday for dinner for mash or something?
They're currently in a bowl of water.

Yes, keep in a bowl of water in fridge. If keeping more than a few hours take out of bowl, wash under running water and put back in bowl with fresh water.
Repeat until ready to use. Important to keep in fridge

Why not cook and mash now, that will definitely keep until saturdaynin the fridge.

Thanks both. At the mo doing as suggested - in water and keep changing it.

I've never pre-made mash. How do I reheat it? When do I add butter etc?
Does it taste just as good?

Pre-made mash does not taste as good. Not too bad either, just not quite as good as fresh. The addition of something like chives or mustard helps.