You seem to have th exact pond situation I have. 4 acres, bass heavy, low BG numbers.
Keep harvesting, we took out 250 between March and April. then added 1000 adult CNBG and 60 lbs of Tilapia.
Now you have to fish a while to catch a bass. So we stopped at 250 for now. It turned out to be about 52 lbs per acre!!!