The type or namespace name ‘log4net’ could not be found (are you missing a using directive or an assembly reference?)
When creating an application with dotnet 4 (dotnet4) there is a caveat regarding log4net. One has to change the Target Framework from the default .Net Framework 4 Client Profile to the full .Net Framework 4.
This is done through the project properties.
A slightly strange side note is that in my VS2010 log4net showed up in the intellisense to start with but disappeared later; until I did the Target-Framework patch.
The tutorial/quickstart I used is here.