Every now and then when I build my solution here (with 7 projects in it) I get the dreaded \'Command copy exited with code 4\' error, in Visual Studio 2010 Premium ed.
As mentioned in many sites, there are various reasons for this. For me it was due to the length of Source and Destination (Path length). I tried xcopy in the command prompt and I was unable to type the complete source and path (after some characters it wont allow you to type). I then reduced the path length and was able to run. Hope this helps.