Most powder scales are set to 0.1 gr resolution, or about 6.5 mg. On a scale that's capable of measuring 100 g, that's better than 0.01%. Analytical scales that can do that reliably come with an enclosure against airflow, are usually set on granite weighing tables against vibration and cost $1000+. So the old question of resolution vs. accuracy rears its ugly head.
Now, 4 gr drift, or 200 mg difference in measuring the calibration weights (assuming you didn't ding one) is bad. My RCBS combo usually is repeatable within .2 gr for repeated measurement of the same bullet. Customer service call seems in order.
F 135 - the right choice