You can still use Gen4 figures in DS4.x. Collision detection and the smoothing modifier will still work with Gen4 figures.
Genesis can also load Gen4 textures, just remember to use the appropriate UV layout. Conforming clothing can be converted via AutoFit and leftover pokethroughs be minimized with collsion detection and the smoothing modifier.
DS3 comes with Elite Human Surface Shader, while DS4 comes with UberSurface which is more capable. You have to buy UberSurface separately if you want to use it in DS3.
Posing Gen4 figures in DS4.5 is slow though, much slower than posing Genesis. Rendering is also slightly slower (using the same lights, settings and materials). Well, at least for me. In the end, I still prefer to work with DS3.