Is it possible fo me to take the terrain asset out of unity and into bender,
then sub-divide it and reimport?
would it work?
would i then have a higher res terrain, and the paint will work all the same?
Not really, since the terrain is a heightmap, not a mesh. (There are scripts which export the terrain as a mesh and convert a mesh to a terrain, but in this case the subdivided mesh would be too big to import into Unity.) You can export the heightmap and manipulate that in Photoshop though.