If you've created a customized player template using EE2SP1 or EE3 and have now upgraded to EE4 in is necessary to upgrade your custom template as well in order to render smooth streaming content. The smooth streaming manifest format has changed between EE3 and EE4 and the older templates cannot render the new smooth streaming format.
The process for upgrading a customized template is straight forward, though perhaps a bit tedious.
1) In EE4 create a *new* custom template -- select the same template when you originally created your custom template in EE2SP2 or EE3 -- use the "Edit copy of template in Expression Blend" or "Edit copy of template in Visual Studio" commands.
2) Rebuild the new template and verify that it builds correctly -- It is also good at this point to verify that the rebuilt template will render content emitted from EE4 -- use encoder to encode some of the Windows sample content (Bear or Wildlife) using the encoding profile you plan to use.
3) Using a "diff" tool -- such as "Beyond Compare" merge your XAML and C# customizations from your EE2SP1 or EE3 custom template to the new EE4 template. This is the tedious part. One thing to be aware of here is that in EE2SP1 and EE3 each custom template has a complete copy of the common "media player" source code -- whereas in EE4 all custom template share the same set of common code.
4) Rebuild your new custom template and test with the same sample content from step 2.