You are correct, the OOB SAP connector provided in IDM targets the SAP R3 system. SAP is not my expertise and my knowledge on it is extremely limited. What SCIM specification is supported within S/4 or whether it is supported at all is a question for the SAP forum but a quick Google tells me SCIM 2.0 seems to be supported in SAP.
I would also suggest to reach out to ForgeRock support and see if they are working on S/4 connector by any chance. If not, the SCIM connector is likely your best bet or rollout a custom.