Regarding duplex loads, what I've read and from my experience is that most hold to 10% of the BP.  This part is solely from my experience with double rifles, mostly and a bit with single shots.  I have never needed more than 5% to achieve what I wanted.  My first experience was with an E. Goldmann German double rifle from the 1870's in 11.15 X 60R.  After 6 months of  juggling bullets and powder charges and granulations I simply could not generate enough velocity to get the barrels to regulate.  5 grs. of SR-4759 under 67 grs. of Schuetzen Ffg finally brought them together and it shoots exceptionally clean as well as quite accurate for a 150 year old rifle, + or - .  I had similar results with an E.M. Reilly double in 500 BPE.

In single shots the two cartridges were a 45-100, (2.6) and a 45-120, again using SR-4759.  At that time I didn't know as much about fouling control as I do now and ended up duplexing both cartridges with 5% of the BP charge with satisfactory results.

So, yeah.  Back off to that 4 grs. and see how she does.  I hope you're pleasantly surprised.

And I sure wish they'd bring back SR-4759.