Any and all carbs. Very little fat, moderate protein and LOTS of carbs. Ideally on these 'refeeds' you should aim to take in from 110% to 120% of your maintenance calories as mainly carbs. Simple, high GI or lower GI stuff...doesn't matter (though higher GI is slightly better). This assumes that you're dieting pretty strictly the rest of the week.
I have my diet set so that I have a small cheat everyday. I guess thats not a cheat. Something sweet. I could see how you could save them and use them all in one day.
I just have a fear that one day of 4000 calories aren't a good pay off for a week of hard work.