I am not sure if you can get nearest neighbour on the rendering without redoing some of the rendering code in C++. The best solution that would be relatively simple, would be to set the resolution to 640x480 and use a post process material.
There are a few varieties of post process materials when it comes to Unreal, as well as tutorials. Here is one I have for sale which has some cool extra stuff: Elias Wick | Pixelate Advanced