I do want to stress that the twelve person games are a design decision rather than a technical one. That’s where we’ve found the abilities remain manageable and deliberate, it gets very chaotic as the player counts scales up.
Beyond optimization, there’s a lot of reasons that UE4 is a great choice for us over UE3. 4 has a lot of features that allow a small team like ours to work much more flexibly, even allowing non-programmers to fully prototype or implement gameplay and rendering features as well as complex shaders. The rendering engine is now physically based (very nice reflectivity and better grounded materials and surfaces), and supports some great lighting features that have added some life to our environments.
I’m sure other folks have reasons they enjoy it, but from the art side, it’s visually head and shoulders above 3.