Hi Sander,
you’ll need to switch the nodes on the V3 and v4 project. However Mendix can’t do this directly so you’ll need a third app.
Create a new application, name it migration. Make sure you’re TC of all applications.
Open the migration app and select the V3 node, now open the V3 application and you’ll notice it doesn’t have the environments anymore. Now select the V4 node here. Then open the V4 application and select the node that is currently under the migration app.
This way the nodes are switched from v3 to v4. You should now have all the project information and history in your v4 application
For more information:
https://docs.mendix.com/developerportal/deploy/migrating-to-v4#5-migrating-the-app-manually