Can I prep potatoes now for mashed potatoes later?

You can peel, dice and put in water and sit in the fridge.

When you walk in the door, drain the cold water, replace with hot from the tap and put on to cook. By the time you have changes out of your work clothes, they should have come to a boil. If you diced them small, they should be fork tender and ready to mash in about 15-20 minutes after they started boiling.

When I need to do that, I dice the potatoes small so they cook faster.

Yip they sure will be fine! you can leave potatoes in water for a couple of days as long as they are washed and refrigerated. Try cutting your potatoes into smaller pieces so they cook faster. hope all goes well! :)

I have boiled them, strained them, and left them in the fridge unmashed overnight before. I don't think they will oxidize if you have them in water. My guess is 4 hours in water shouldn't affect them too much.

since you're mashing them afterwards, it should be fine