Hi Marcel,
After applying the transports on SAP S4 system, you have to use a t-code /ARBA/MASTER_DATA(or program /ARBA/MASTER_DATA_EXPORT) on your S4 system to export the master data from S4 system onto a specified file system as .CSV files, after this you can have direct connectivity to upload it to Ariba or setup your PO system to do that for you.
In our scenario we had used PO to take care of that tasks, so our PI communication channel will pick up the files from S4 and place it on PO server and later we will run the ITK job(AribaIntegrationEventFullUpload-can be scheduled on PO JAVA)to upload these files onto Ariba, we had created a folder structure on PO server for incoming, outgoing files and logs for the ITK job runs, please note there is some standard configuration that needs to be done nefore scheduling the ITK jobs on PO server.
Hope this helps.
Regards,
Ravi A