There are many challenges, when we are dealing a Global SAP BI system and four different regional SAP BI systems. Many things can go wrong, with regards to data security/authorization. There will be regional users who need to have access to just one regional data. Some regional users need to have access to global data(all regions). Some regional users need to have access to just 1 or 2 regional data, not all regional data. Therefore I proposed a strategy to manage the same.
Take a look please:
Global SAP BI Authorization Roadmap