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I believe both have to be recreated

Enough has changed about the command strucutre that realistically, customized shortcuts won't make sense to try to preserve. Shortcuts you may be using now in MU3 might be reserved by a new command in MU4, etc. So while it could theoretically be possible to export a shortcut definition file from MU3 then import it into MU4 after editing it as needed to make sure the file itself isn't rejected, it wouldn't be a good idea.

For palettes, also, the default layout of the palettes is changed considerably, and I wouldn't be in hurry to reproduce an entire workspace and thus give up the things added or changed in MU4. But, individual new palettes you've created would definitely be nice to be able to import. I just tried that, and was pleasantly surprised to find it worked, not even a need to edit the palette definition to change a version string or anything. So nice to have my "Annotations" palette in MU4!