4 grains sounds about right under a middleweight cast bullet in the BLK.
6.4 grains of TG is my go-to sub load with 230-grain cast bullets...in 7.62x51mm U.S. -spec machine gun brass. .308 Win brass might need a couple tenths more powder due to the extra room. The same bullet in the tiny BLK case will need a LOT less and as Fiver and others talked about here many times TG gets stupid with pressures real quick if you use too much in a tight space behind too heavy of a payload. With a middleweight bullet 4 grains still should be well between sticking the bullet in the barrel and blowing things up and with chronograph data you can tell real quick which way to go from there. I'm not sure I'd even try TG with heavy jacketed bullets, especially the tough ones designed for the hyper-velocity magnums.