Keep in my that a lot of factors will change the look of a material in UE4…
All materials you see in Bridge or Mixer are tessellated, thats something you can replicate in UE4 with a tesselation or POM Material.
Good lighting will also contribute to a better look of your materials in UE4, use all common lighting techniques to achieve your desired look. You will find a ton of toturials on youtube, from simple lighting up to HDRI lighting in UE4.
Last important note, UE4 is using TAA as standard antialiasing methode. TAA does a good job, however it will blur you textures so you wont get/see the same level of detail as in the mixer or bridge previews.
Something i forgot to mention, keep also your pixel density in mind. If you are using 2x2 Meters Scans on a 10x10 meters surface, without tiling your material, of course it will look horrible.