Err. Uhh. The Unreal engine scales to MMO-capability by itself.
I don't know if anything has changed with UE4, but the last time I worked with the Unreal engine, it didn't by far.
Unreal is designed as a first-person shooter engine and its built-in networking code is based around that. It's designed for low-latency medium-bandwidth links to support twitch gaming. It has a hard player cap that was originally 16 but I think increased to 64 in UE2 or 3 (the stuff I used it for never came close to that). The out of the box server will let you load statelessly connect several players together on a single map. If you want more than one map, you have to run multiple servers and there's no built in method to persist data across them or control who connects to which one.
AFAIK it doesn't have any kind of seamless transfer support that people have come to expect in a modern MMO. So you're looking at frequent loading screens / waiting for connection messages when moving between areas. UE3 added "Seamless Travel", but that's for a local game or a co-op server where the server itself loads a new map because everyone on it is in the loading zone and moving the same direction.
There's a couple of methods to use it for an MMO instead. You can either use the built-in networking code and write / use your own stuff on the back end to handle character state persistence and moving them between maps. That's what DCUO does IIRC. It's decent for games that need fast reaction to things, but if your MMO isn't twitch based then you're wasting a lot of bandwidth (which adds up fast in the datacenter with thousands of people connected).
The other option is to use a completely custom server backend, eschew the built-in networking code altogether, and replace it with your own. In that case, Unreal would think that it's a single player game running locally, but your own code plugged in to it is creating actors that seem to be computer controlled, but are really being driven by a connection to your custom MMO back-end. With that setup, you have no limits on the number of players in the area beyond what your own code and the client PC's graphics can handle. You *might* be able to leverage the Seamless map loading support in this setup, but it would be tricky to get right.