In 4.0.1, the syncSourceId for a node identifies the member of the replica set the node is syncing from. Starting in 4.0.2, the syncSourceId for a node doesn't change unless the node is either stepping down or stepping up itself.

Additionally, if chaining is disabled there is no logging explicitly stating the new sync source when there was in 4.0.1.

4.0.1:

2019-09-24T18:12:41.842-0400 I REPL     [replexec-2] Member localhost:27018 is now in state PRIMARY
2019-09-24T18:12:42.335-0400 I REPL     [rsBackgroundSync] chaining not allowed, choosing primary as sync source candidate: localhost:27018

4.0.2:

2019-09-24T18:04:18.868-0400 I REPL     [replexec-0] Member localhost:27018 is now in state PRIMARY