Assuming this is a 12ga with that light a load I think the first thing I would do is switch to 3FG powder. 4cc of powder is not much over 2 drams & with less than 7/8oz of shot I think the 2FG is likely simply too slow burning to build enough pressure to burn any cleaner. I realize of course BP is not affected by pressure as much as is smokeless, but still suspect with so light a load you would benefit by a finer granulation.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra