I just found a very useful XSD for editing log4net configurations:
Just copy the schema file to C:\Program Files\Microsoft Visual Studio *\Xml\Schemas and add the correct namespace to the log4net element in your config file:
<log4net xsi:noNamespaceSchemaLocation="http://csharptest.net/downloads/schema/log4net.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
/Emil