General

Do not upgrade in a live production environment. EPiServer4 is almost completely rebuilt from the ground up and is entirely based on the Microsoft .NET platform. The only thing that is left from EPiServer 3 is the database structure. We recommend extensive testing of customized templates and other vital functions. Special functions for editing may have to be adjusted. Se below for limitations and known problems.

Limitations

• Workflows will not work. Workflow is implemented in EPiServer4.10, but it is not compatible with workflows made on EPiServer3.
• Customized properties in 3.x will not work unless they are rewritten for .NET.
• If you update pages with the API of version 3.x they will not be reflected in version 4 until the cache has been released, which is set by default to 1 hour upon the upgrade from 3.5 to 4.0. For example, this applies to customized edit pages and forums. Updates that are made with the API in version 4 will be visible directly both in version 3 and 4.
• The Right Click Administration (RCA) will not work with EPiServer 4-versions prior to release 4.10. You need to download a new version of HkaInc.asp and replace the existing file. 
  If you have upgraded to EPiServer 4.10, 4.11 or 4.20, download this file.
  If you have upgraded to EPiServer 4.21 or later, download this file.

Known problems

• A new installation of EPiServer 4 will be installed on top of EPiServer 3. This means that if  filenames (on templates, pictures, stylesheets etc.) collide, they will be replaced. For example, if the stylesheet used is the default "episerver3.css" it will be replaced  because the default stylesheet in 4.0.0.18 is also named "episerver3.css". The stylesheet is named "episerver.css" in version 4.0.0.19 and later.