Probably involved in the priming of starch granule formation. May play a regulatory role in the control of starch accumulation in plastids. Is necessary and sufficient to establish the correct number of starch granules observed in chloroplasts. [(1->4)-alpha-D-glucosyl](n) + ADP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + ADP + H(+) Glycan biosynthesis; starch biosynthesis. Interacts with PTST2. Expressed in leaves and flowers. Severe reduced growth, reduced number of starch granules, altered structure of starch granules. Plants over-expressing SS4 have increased levels of starch in leaves and display a higher growth rate than wild-type. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.