I would imagine someone will confirm that setting Max Texture Size to use both X & Y values is not going to work, Square textures are more memory efficient as they tile easier. Given a texture that is wider than it is longer or vise versa and not square the engine will have more of a harder time with it than a square texture of the same resolution. This is why in game dev we use figures of 128x128, 256,256, 512x512, 1024x1024, 2k, 4k, etc. 
4k is a ridiculously large texture if you plan to run on Android. Some android devices cannot even render larger than 2k resolution. Perhaps you should reconsider the texture scales and assets in your project. Or port the project to a new uproject, get stripping for an android compatible version.