Firstly, to get the terminology correct, it's "organisation", not "instance". So you currently have 1 Adobe organisation where your 4 report suites reside in, and also have 1 AEP product provisioned under it.
Now, if you were to have 1 Adobe organisation per region, then you would need to replicate your setup across the 4 organisations, particularly if you want to make full use of the UI integrations in AEP Tags with Schemas and Datastreams (for the Web SDK setup).
You could continue with 1 Adobe organisation, but it would require a lot of custom coding and mapping of Datastreams + Analytics. I personally don't recommend that, because it's prone to human errors.
Besides, when a business wants to separate its products in the way you've described, it usually means that there is going to be a lot of uniqueness between each website, e.g. website A wants to track eVar 100 but website B doesn't need to. So again, it might be better to take the pain of separating them now, with the expectation that each will really become its own set.