Hi Aleem,
I have reviewed my documentation here, and for the case that you have mentioned the solution would be the use of Side-by-Side Extensibility on HCP (Extensibility of S/4 HANA).
S4 HANA was designed taking into consideration special needs, and they are using Hana Cloud Platform to cover these specifics situations.
Please, check the slide below from Week2 Unit 4 from S/4 Hana Deep Dive course.
The integration of S4 HANA with Ariba is native and probably it won't be a big deal to solve.
Thanks for sharing your questions, I think that is the best way to improve our knowledge about S/4 HANA and its capabilities.
Regards,
Marcelo Rodrigues.