Zbrush painting is assigning colour to the vertices of the mesh, so the rough rule of thumb is 1 poly = 1 pixel. So a 4096 x 4096 pixel map = over 16 million polys. Then factor in how much of the UV space the model occupies - if it's 60% then your minimum poly count would need to be in the region of 10 million. (This is based on advice from Zbrush Central docs and Scott Spencer's book.)

V4 for example has three main maps covering head and body - to paint and generate a detailed 4k set of maps would take more polys than my system can handle (32Gb RAM) so I would have to use HD geometry. The advantage is poly count into the tens if not hundreds of millions but you then lose the ability to paint using layers and can only paint smaller areas of the mesh at once.

So the map size and way that the mesh is UV'd will indicate the resolution needed to get good maps. The advantage to vertex painting is that you don't need UV's until you want to turn the paint into a texture map; the disadvantage is system resources required for larger map sizes.

As usual, different apps and workflows have their own pros and cons.

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