Hi Tansu,
first, you need to distinguish different deployment options: S/4HANA on-permise is different from S/4HANA public or manage cloud. Cloud solutions are much more restricted.
Secondly, you need to take the release planning into consideration: currently S/4HANA includes only Simple Finance but time flies and soon more and more "Simple" components become available. All this is described in the roadmap and needs to be mapped to the customer's project timeline.
Third, taking "Simple Finance" as an example, the installation is not a classic add-on but a so called "Exchange Innovation" which changes and/or removed SAP code and database tables! You also get Fiori apps... So functionaly these are not the same. (This is quite apparent if you look at the impact on custom code, see cookbook).
Have a look at the Cookbook,
http://scn.sap.com/docs/DOC-64980
Regards,
Marc Bernard
Product Management SAP EDW (BW/HANA)