The migration issue has been found and will be fixed in Drafts 4.0.2, which will likely be submitted to Apple on 10/16.
Some users are reporting an "Import Failed" error trying to migrate drafts to Drafts 4.
It appears some users have some history in their drafts that was not encountered in the beta cycle and is causing an issue with the import. If you would be willing to share your export file for testing, please follow the migration instructions, but select "Email" as the destination instead of "Open in" and send it to info@agiletortoise.com.
If we have a few of the problem export files, we should be able to sort this out quickly for an update.
Be aware that migration is additive, so feel free to go ahead and use Drafts 4 and you will be able to import your drafts later when this issue is resolved.
Thanks for your patience!
I've had that issue, and I've just emailed you my exported JSON.
I have to say I exported from iPhone and tried to import on both iPhone and iPad the same export.
On the iPhone worked, on the iPad I got the error.
Hope it helps.