hi -2 exposure should be corrected on your camera itself, has nothing to do with your +4 diopter, as diopters have no light-reducing quality (unlike, eg. cpl).
for diopters, and macro photog in general, its harder to focus the bigger the image, as the DOF will get smaller and smaller. so for max magnification with your setup, you set your sigma to 300mm, put manual focus for the lens and move it to the minimum (opposite side from infinity), and then move closer and further from the object to bring it into focus. warning: the DOF will be PAPER THIN. so be careful/use tripod.
try to use high apeture like f22. turn your flash to TTL and pull out the bounce card to get max light on the subject close to you. this will only work on higher focal length. i used my di622 on a 50mm with +4 and the bottom half of the frame was not illuminated, but at 200mm +4, it was fine.
hope this helps