It's all in the shaders and The light. So many artists forget how big of a change lighting can do, i'd recommend looking up Tilman Mildes UE4 Lighting academy on Youtube (and UE4 forums) listen to his advices! =)
While UE4 and Unity are difference engines, a lot of basics are very much similar. For the terrain texturing painting, you need more textures. I see what looks to be 1 grass, 1 dirt, 1 sand and 1 rock texture. I would say at least 2-4 of each. You can use the Brush Opacity and Target strength to blend between the textures…