Hi,
We are using log4net everywhere and have had no problems with .NET 4 projects referencing it. I just removed the reference and added it again in one of my cusotm projects and I did not get any errors.
What steps are you using to add a reference?
The way I do it is right click the references node in my project and choose add reference, then I click the Browse tab, and then I browse to the log4net.dll in the _libs folder included with the mojoPortal source code.
Hope that helps,
Joe