Hi Ardy,
My recommendation is to create at least 4 separate standard DSOs (Propagation layer) with key fields which correspond to the source table(s). Standard DSOs are much easier to handle in data staging processes.
Optionally, you can also create 4 write-optimized DSOs (Corporate Memory layer) for emergency cases.
The Transformations can indeed be implemented from the Propagation DSOs to the Consolidated DSO (Business Transformation layer).
Re. the aggregation, I think that you should experiment with the Transformation. You can oversee best how the aggregation should be done, try to achieve the required aggregation behavior by evaluating Business Content and projecting it to your new data staging model.
Best regards,
Sander