Hope you have worked through this problem by now, For me, I say a four grain variation is not even close to acceptable. It's possible to make runs of bullets with a fractions of grains variation. To give an honest appraisal it would be necessary to see the bullets.

Casting technique: Possibly, the first place to look is the sprue. The lead is drawn in to fill the mold. Allow the sprue to cool enough to change color. This is where the lead is drawn in to fill out the bullet. The bullet base may have rounded corners. I cast at 750 degrees. To me. when casting bullets everybody stays a newfer. There is always something to learn. Don't give up. Uniformity has all to do with technique.