The problem is the namespace:
Your message has a namespace of: http://BizCompTest2.BizCompTest3OutSchema
While the xpath has a namespace of http://BizCompTest4.BizCompTest4OutSchema
The xpath namespace is set when you promote the distinguished field in your schema and will match the schema target namespace.
You need to change the namespace in the second line of your code above
Also, if you set the "Use Default Constructor" property on xmlDocVar you do not need the first line.