What I would advise you NOT to get is the 4x4" (square) "LEE Filters 4x4" Circular Polarizer Glass Filter" ... I believe that these 4x4" square polarizers (linear or circular) are used mainly in video & movie matte-boxes, and you would not be able to control the orientation of your ND-grads & the degree of polarization at the same time...
Sure you can. You need a second Foundation Kit and the Lee Tandem Adapter. The 4" square CPL goes in the second holder, the tandem adapter lets you join the two holders, and you rotate the outer holder.
The combo of 4" CPL, holder and adapter is cheaper than the 105mm front ring plus 105mm B+W CPL. The former setup is thicker, so the more expensive latter combo might be worth it if vignetting is a concern.
Cool! Never knew that, thanks for the info!