first refer here
HCM Solution options for S/4 HANA
https://websmp207.sap-ag.de/~sapidp/012002523100018395812015E.pdf
1. Once he is migrated to SAP S4 HANA, does he need Enterprise Portal again, if yes, what will be the possible landscape of the same.
Are they moving the entire system to S4AHANA, Ie upgrading to 1511?
2. If the client has to continue with Enterprise portal, does he need to upgrade his Netweaver portal. If yes, what is the most preferred version.
He can continue using HCM on Premise as is with integration with S4HANA
3. What would be the most preferred Go-To strategy if he has to continue with ESS/MSS
Integrate with S4HANA
having said that please note
There is nothing planned for ALE employee replication.
For ALE the customer need to transport his customizing 1:1 to S/4HANA. and need to build their own for ALE replication in 1511, its in the roadmap for yearend
Read here
SAP S/4HANA customers also using SAP ERP HCM on-premise and might not be willing/able to move to SuccessFactors in the near term and like to continue to use SAP ERP HCM. SAP HCM can be run in either a separate instance or single instance with SAP S/4HANA. The integration among both on-premise instances (SAP HCM with SAP S/4HANA on-premise) will be available via ALE in case SAP HCM is running on separate instance. In case SAP HCM is running inside of SAP S/4HANA on-premise then the integration is automatically given. 2.1.28.1.2 Business Process related information The Software components SAP-HR and EA-HR are still available in the SAP S/4HANA 1511. The go-to architecture in HCM area is SuccessFactors Employee Central. It´s not planned to provide application simplifications based on the traditional SAP HCM functionalities. As example: It is not planned to offer Fiori based user interfaces for these traditional functionalities. SAPGui based functionality stay unchanged for those customers unable to make the move to SuccessFactors in the near
You can refer
SAP S/4HANA, on-premise edition – SAP Help Portal Page
Let me know if you have further queries