Borderline inferior cerebellar ectopia present, the cerebellar tonsils demonstrate mild inferior herniation, measuring 4 mm sagittal sequence, means?

It most likely: doesn't mean anything. If that is the only finding, and the tonsils are otherwise normally shaped, some use less than 5mm as the normal range, so by that criteria 4mm would be normal. Others call 3-5mm benign tonsillar ectopia, an incidental asymptomatic finding. Most consider greater than 5mm to be abnormal(Chiari 1 malformation). The measurement is from the skull base to the tonsil tip.