foxrever Unfortunately, it's not possible to select multiple bones with a single operation. However, I noticed something I overlooked in your original question. You want to bind the bones that are bound to a mesh attachment to a bounding box attachment, correct? If so, you can use the copy and paste function in the bones list of the weight view. This has been possible since version 4.2:

I hope this helps.

foxrever

The error you’re seeing is a bug—we’ve just patched it, and the fix will be available in the next beta.

However, that still won’t allow you to import adjustment layers.

To work around that, you can convert each layer or group you want to export as an attachment into a smart object, and apply the adjustment layer inside it.
That said, if you have too many layers, this might be too tedious. In that case, you can use the method Misaki suggested above.

foxrever Sorry for the confusion. Unfortunately, the Chinese user guide has not been updated yet, but the adjustment and fill layers are explained in the Limitations section of the Import PSD page of the user guide:
https://esotericsoftware.com/spine-import-psd#Limitations

As mentioned there, a workaround is to use the PhotoshopToSpine script. Since this script is for Photoshop only, it can handle some things better than the PSD import function available in the Spine Editor. (It can handle PSDs exported from various image editing applications, not just Photoshop, so it will not handle some Photoshop-specific layers or features).

There is an issue ticket here for improving the Spine Editor's PSD import feature, but since there is a workaround using the PhotoshopToSpine script, this feature is not a high priority:
EsotericSoftware/spine-editor861

@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

foxrever Unfortunately, the current functionality does not allow you to keep the constraint settings when moving bones from one skeleton to another. We are already aware of this problem and mention it in one of our issue tickets : EsotericSoftware/spine-editor582

However, since Spine 4.2, it is now possible to copy and paste the constraint settings, making it much easier to reconfigure the constraint. (Copying and pasting settings works well for different skeletons as well.)
In the tree view, select the name of the constraint whose settings you want to copy and press Ctrl+C. Then select the name of the constraint you want to paste and press Ctrl+V.

Yes, you can use animation clean up. English:
http://en.esotericsoftware.com/spine-keys#Clean-Up
Chinese:
http://zh.esotericsoftware.com/spine-keys#%E6%B8%85%E7%90%86