To upgrade to Retain 4 requires that Retain be upgraded to 3.5.1.1 first. Please see the Planning and Design Best Practices page for system design recommendations.
Retain 4 requires new licenses to function after the upgrade. An upgraded Retain 4 system will not function until the licenses are applied.
Upgrading from earlier versions of Retain follow the general plan of: 1.x to 1.8, 1.8 to 2.6.2, 2.6.2 to 3.5.1.1. This will probably also involve operating system and database upgrades as well.
Once Retain is updated to 3.5.1.1, the upgrade process to Retain 4 may begin.