You should be able to do 4 inches with the right reload, yes.

That is if you do your part !!!

Firstly, do you now the factory load velocity ?

Secondly, do you have a Chronograph ?

Thirdly, what does it shoot with factory and your loads.

4, what other changes have you made / tried in terms of different powders ?

5. How are you shooting it ?
Standing, sitting ?

Also, what range are you shooting at. Get everything 100% at 50 yards first before going to 100 yards.

6. What target are you using and where are you aiming ?

Are you using an 8 inch Black and aiming for the centre bull ?

If so, I would suggest putting a 1 or 2 inch White Circle (Sticker) at the 6 o'clock position and use that as it is easier to ensure the same sight picture for BOTH shots.

Hope the above helps and isn't teaching you to suck eggs,
sorry if it is !!! LOL

Good luck.