I’m doing this. So far using particle systems with positioning of the particles primarily in the material shader (using WorldPositionOffset). There are forum threads about this around here somewhere but I don’t have them at hand right now. This works quite well for a few hundred thousand particles but I’m pushing it further and I’m looking into additional options. One route might be to turn it into a volume rendering - I have not really tried this myself so far. If you’re interested in Volume Rendering you can look into these threads:
https://forums.unrealengine.com/development-discussion/rendering/91596-your-thoughts-on-and-comments-to-volume-rendering-in-unreal-engine-4