Now the only issue with Substance painter, is if you want to export 8k maps, I think it is, you need at least 4 GB memory on the graphic card if I recall correctly.

Yes you need a lot of memory in zbrush too to extract the normals out of the mesh. This is due to the fact that it is subdivided a lot and the details are calculated on the lowest sub and then extracted and placed on the UV map/normal.