Make sure sRGB is unchecked for the metallic, roughness, emissive, and ambient conclusion textures.
Test using the Advanced Lighting Map, it has an HDR image setup like Substances’ viewport. It’s a good place to start testing assets before you build a scene in UE4.
Your normal map looks inverted, try inverting the green channel or making sure it’s set to DirectX in the Substance settings.
I think ideally you should not plug 0 into Specular, the default is a magic number. Also make sure you have a reflection probe in your scene as it looks to like you have none and without one you will get that super flat look, PBR relies on reflections to work correctly.
What Anadin said about the Specular being zero is correct. For metal/roughness workflow, .5 is the mathematically and physically correct value to adhere to realistic lighting. You’d only ever ever change it if you know what you’re doing and are trying to achieve a specific, unrealistic, effect.