There's little evidence around the internet at the moment, other than that published by Microsoft, which can be found here and here, which states, to summarise:
Several changes have been made to the Silverlight runtime and the
Silverlight Tools between Silverlight 4 and Silverlight 5. For these
changes, the following principles apply:
Most Silverlight 4 applications will work with Silverlight 5 without
any changes.
When breaking changes are required, Silverlight will try to maintain
support for the old behavior, as well as the new behavior, by using a
quirks mode.
Nevertheless, some changes made to Silverlight components can
potentially cause your older Silverlight-based applications to fail
(compile time, XAML load time, or possibly design time) or to behave
differently.
And,
There are no known breaking changes between Silverlight 4 and
Silverlight 5 in the Silverlight core runtime. Applications that were originally compiled using Silverlight 4 tools, and that continue to target the Silverlight 4 runtime, should work without issues on a client that has the Silverlight 5 runtime.
If your project references any Silverlight SDK client assemblies [...] make sure that your project
specifically references the Silverlight 5 version of the SDK client
assemblies [...]. A project that targets Silverlight 5 cannot use the Silverlight
4 SDK assemblies.
There's more information in the links I provide, for instance, related to quirks mode, third party references, and behavioural changes.