When you are planning to upgrade then you don't have to uninstall and reinstall the software.
Regarding the upgrade instructions, Please refer to ePO 4.5 installation guide. Page 20 talks about upgrade from ePO 4.0
In addition to this, I will suggest you to follow ePO 4.0 diaster recovery document, KB#:KB51438.
The KB talks about migrating the ePO to new hardware but please backup the database, ePO folders as per this KB.