To see the location of the current version of node you are using:
nvm which current
You are using the system installation if .nvm is not in the path, similar to the following:
/usr/local/bin/node
To switch to a version managed by nvm:
nvm use 4
To verify you are using a version managed by nvm:
nvm which current
You should see something similar to the following:
/Users/<your-user-name>/.nvm/versions/node/v4.2.2/bin/node
You should only experience global install permission issues when you are using the system installation.