Let's now run a FirebirdSQL installation on a development machine with both FB3 and FB4 versions, why do I mention only development? It is not recommended in the same environment two (or more) different versions of the same RDBMS in the production environment because the performance and backup significantly affect the entire environment and in addition, in the event of a disaster, the “disaster recovery” has a much greater amplitude. But in a development environment where the connection is usually local (remember, local is not localhost) it is perfectly safe and recommended.
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