The 940m isn’t much of a gaming chip, but in practice it will give you up to 80% higher framerate. For UE4 that is well worth the slower CPU. You will also get an extra 2 GB of total RAM, and the CPU might be able to turbo boost more, as there is no heat generated from the shut off HD 520.
256 GB SSD is enough for plenty of UE projects as long as you do not also install lots if games.