I would imagine that Phase III will last some time: the current software
base is very robust and extensible, and can take lots more features
without major re-architecture.
Even moving the database and various front-end servers to different
machines is still possible with the current Phase III software. (By the
way, this is a good reason not to consolidate all the national databases
in a single database, as we will only have to unpick them later on for
scaling reasons.)
Phase IV will presumably mark the move to a distributed cluster
architecture to cope with the sort of load that overwhelmed Britannica
when they originally released their entire content for free.
Neil