4" separation is really nothing - if you are speaking of 4" between the centres of the groups. Since a proper double, regulates parallel, not into the same group, this spread is only about 1-3/4"(if between group centres. I'd expect that 160gr. might shoot parallel which would be perfect.
: I'm sure the Swiss BP will work well, as it's supposed to give the same velocities as English powder of the mid to late 1800's. It isn't moist burning though, but will give approximately 20% greater velocities and pressure than normal GOEX BP gives. This should allow dropping the charge (and recoil) but maintaining the same ballistics as before.
: Imagine getting the same ballistics from 140gr. as with 180gr. THAT would be a nice occurance.