Stitching the bracketed exposures separately as three sets of four will not result in identical panos.
The control point detection system is very sensitive and will not necessarily find identical CPs at different exposures.
If you what to produce thre separate panos from your bracketed exposures - not sure why you would - then I think you probably need to use hard linked stacks and output by layers.
But perhaps I have misunderstood your question?
Andrew Stephens Many different Nodal Ninja and Agnos pano heads. Merlin/Panogear mount with Papywizard on Nokia Internet tablets. Nikon D5100 and D40, Sigma 8mm f3.5 FE, Nikon 10.5mm FE, 35mm, 50mm, 18-55mm, 70-210mm. Promote control.