Hello,
There is no definite answer to this question. It really depends upon your needs.
You must know that each modality will record its identity inside the "Station Name" tag (0008,1010). So, if you use a single Orthanc server, you can isolate the DICOM images from each modality by filtering this DICOM tag. Any DICOM viewer should allow you to apply such a filter.
You should not be afraid of storing all the images inside a single Orthanc, as using the PostgreSQL plugin would allow to store large amount of data, while providing enteprise-ready features for backup.
That being said, using 4 different instances of Orthanc also makes sense if you need isolation between the different modalities.
It is technically feasible to create one front-end Web server that would present the unified content of each Orthanc server, but this will require development on your side, by creating an external application (in PHP, Java, .NET, Python...) talking with the Orthanc servers through the REST API [1]. This is one of our long-term projects, but we unfortunately have no funding to launch it now.
HTH,
Sébastien-