I am assuming that your 4 databases are related to four applications and the use wants all of them to be merged as one. This requirement is a common one, since this reduces the required number of Doc CALs. If this is the scenario, one has to understand that Doc CALs might not work for a model that has big(subject to number of records and tables it has) multiple disconnected data islands. Therefore, if you can make an association among the models (i.e. - they are connected), you can logically go ahead with the model. Now comes the data size and performance question. For that, refer to the earlier link.
