Just a quick video testing out a Sigma 35-70mm f2.8-4 canon FD mount I found at a local camera shop for $20. It seems to work better as a taking lens than my canon FD f1.4 for some reason, not sure why. It does have a flat front element. From what I have read online most people weren't very happy with sigma zoom lenses in the FD mount, as far as the overall quality. it was all shot wide open around 39/41mm except for the 3rd shot which was at 70mm.
I had heard you could use a 35mm + anamorphic on the gh4 without any vignetting but it is pretty severe when I try.