When you use Rotobrush the footage should be at 100% scale and the frame rate must match the original footage. After Rotobrush is complete you can scale up the layer to 4K. Somehow I don't think this is what you are asking. Please explain your workflow in more detail.
so i was using the roto brush tool in a 1080p composition, i was told by the director of the video i had the correct version and recently the director told me i had the wrong footage and gave me the 4k (original files) to roto and it took me a long time to roto and i really to transfer the data from the 1080p composition to the 4k version.
You cannot transfer the data. At best you can render out a matte and apply it to the higher-res footage, but of course then there will be artifacts. you realyl should not waste tiem and simply get do it and redo the roto.
There is one thing you can try:
Render the matte out (as Mylenium suggested) and then apply the Refine Soft Matte effect to it.
Refine Soft Matte is working like the Refine Edge Tool of Roto Brush, but can act on any matte.
I use this technique in this tutorial (in combination with a mocha track mask, but again - it works with any alpha channel, no matter whether it is from a mask or a matte)