Hi Marcel,
let's talk business first: What is the business process (or part of that process) that you would like to integrate? This will determine what type of interface makes sense to use. For example, if you wanted to integrate Ariba Network for the Purchase Order to Invoice process with SAP S/4HANA, there is an S/4HANA scope item pre-defined for this. You'll find it at http://service.sap.com/hybrid> Procurement > S/4HANA, or for the on-premsie edition of S/4HANA go to http://service.sap.com/bp-s4-ariba and for S/4HANA cloud go to http://help.sap.com/saap/library/factsheets/J82_S4HANAX_EN_DE.htm.
With S/4HANA, no middleware is required, the integration with the Ariba Network is natively built into S/4HANA. Planned scope items for S/4HANA integration shall address Invoice to Payment on the network, and Sourcing.
If you were going for integration with SAP HANA, on the database level, you may want to consider using the SAP HANA Cloud Platform, with its built-in integration services, formerly known as SAP HANA Cloud Integration (HCI), as a mediator and connector. Check out this blog, too: http://scn.sap.com/community/developer-center/cloud-platform/blog/2016/04/29/sap-hana-cloud-platform-integration-services-when-to-use-which-tool.
As an SAP Ariba partner with ambitions to develop own applications on top of Ariba and SAP HANA you may want to check out this partner page on SAP HANA Cloud Platform and the apps you can build with it: https://www.sapappsdevelopmentpartnercenter.com/en/get-started/cloud-applications/
Please let us know your scenario and we can probably provide for more concise support. Thank you.