Installing Windows 7 without DVD/Flash Drive - step 6: no permission?
One of the commands, given in step 5 that prevents this error message from occurring, is repeated  below.

sudo  chown  $USER  bootcamp*.vmdk


This command changes ownership from the root user to the current user. In your case, I assume the current user is named Nick. To verify you entered this command correctly, you can enter the command below. Here I assume your current directory still is ~/Documents/VirtualBox.

ls -l bootcamp*


In your case, the output should appear as shown below.

cd ~/Documents/VirtualBox
ls -l bootcamp*
-rw-------  1 Nick  staff  37376 14 sep 15:43 bootcamp-pt.vmdk
-rw-------  1 Nick  staff    682 14 sep 15:43 bootcamp.vmdk


From your comments, I assume this is what you currently would get.

cd ~/Documents/VirtualBox
ls -l bootcamp*
-rw-------  1 root  staff  37376 14 sep 15:43 bootcamp-pt.vmdk
-rw-------+ 1 root  staff    682 14 sep 15:43 bootcamp.vmdk


This output shows you are not the current owner of these files. Additionally, the + character indicates you have added extended security information such as an access control list.

Another one of the commands, given in step 5 that prevents this error message from occurring, is repeated  below.

sudo  chmod  go+rw  $PARTID


In step 5, a warning is given, which is repeated below.


  Note: This access granted by the chmod command will only last until OS X is rebooted.


BTW, the command without the variable is given below.

sudo  chmod  go+rw  /dev/disk0s4


One way, to verify you enter the chmod command correctly, would be to enter the following command. 

ls -l /dev/disk0s4


In your case, the expected output is shown below.

brw-rw-rw-  1 root  operator 1, 4 14 sep 15:38 /dev/disk0s4


The most likely causes of your problem are given below.


You are not using the default bash shell.
You did not enter the chown command.
You did not enter the chown command correctly.
You were the root user when you entered the chown command.
You added extended security information to the file entries.
You did not enter the chmod command.
You did not enter the chmod command correctly.
You rebooted the operating system.