I'm not an expert with keying by any means but I've had good results rotoing 4K for downsizing as follows:
- Create a 4K comp with the element to be rotoed.
- Bring the 2K BG into the 4K comp and scale it up to the 4K frame.
- Do the roto on the 4K, paying attention to how the edge looks over the scaled-up 2K.
- When you're happy, render out the rotoed 4K with an alpha, bring it into the 2K comp and scale it down to fit.
Doing it this way has given me some very nice clean comps that take account of the interplay between BG and FG. However, when the rotoed 4K render gets scaled down the edges get sharper so that might not be what you want - in that case, scaling the 4K down in a 2K comp might be your best bet - either way, I've always found rotoing over a relevant BG plate (so you can see what's going on with the edge feathers and moblur etc.) is the way to go.