I wrote a long explanation about this, but it was filtered out as spam. This is a short version.
Just want to warn everyone to backup your server before trying to migrate to v4 according to this guide, even the dry run or single site test.
I tried to migrate only one site, but all the sites broke.
Luckily, I backed up the server and could restore it afterwards.
Please EE Team, edit the guide and strongly recommend people to backup the server, even when trying dry run or single site migration.