@foxrever Do you mean that the longest side of a single attachment image is 4800 pixels, or that the resulting atlas is of size 4800 pixels? If it's for a single attachment image of an image sequence, that's quite a huge resolution. Please be sure that you really need this high resolution and are actually displaying this texel density on screen, covering a full 4k screen. If you are not using it 1:1 on screen, it would be much recommended to use smaller resolution attachment images to save both memory and computation cycles.

Regarding requirements for usage at runtime: You can't just copy attachment images into the assets directory, you need to use an atlas so that the spine-unity runtime knows where each attachment texture region is located. If you want to split the atlas into multiple parts you can simply set the Pages Max width and Max height settings for atlas texture packing as documented here:
http://zh.esotericsoftware.com/spine-texture-packer#%E8%AE%BE%E7%BD%AE