Use the Transfer Systems option to move systems from an old ePO - On-prem 4.x server to a new ePO - On-prem 5.x server.
You might see the following error when you register the servers and enable the Transfer Systems options with Automatic Sitelist Import:
ERROR: Master agent-server keys must be imported into the remote server before importing the sitelist. Go to Server Settings to export security keys from this server. Visiting this link now causes you to lose any unsaved changes to this registered server.
Both keys (1024 and 2048) must be imported for successful registration so the Automatic Sitelist Import can save without issue.