J. Warren (Warren1983, pp. 41-43) generally dates Group 4 chalkoi with the legend ΣΙ after those with Σ but observes a period of overlap. She speculates that historical events at the end of the 4th c. or around the second quarter of the 3rd c. might have occasioned the shift.
“KC053 (Early-Middle Hellenistic bronze coin of Sikyon).” In Kenchreai Archaeological Archive, edited by J.L. Rife and S. Heath. The American Excavations at Kenchreai, 2013-2024. <http://kenchreai.org/gaek/kc053>