You do not have to resize your textures at all. They can all be 4k or even 8k resolution when you import them into the engine. Double click on the texture and look under the second section Level of Detail. Just change the lod bias until it re-samples the texture to the resolution you want. It will permanently change the texture to the specified size when cooking/packaging your project, so it will also save project size and amount to the same thing as manually resizing the image.