If you are having problems installing Ionic 2.0.0-rc.4 you may want to check out this issue. I had problems myself and could only get everything working by uninstalling cordova as well as Ionic. So the commands I ran were:
npm uninstall -g cordova
npm uninstall -g ionic
sudo npm install -g cordova ionic
All works fine now….
I’m a mobile applications developer based in the UK, concentrating primarily on hybrid application development with Ionic but also with native development skills. Please visit www.crossplatformsolutions.co.uk for more information about me and how I may be able to help you with mobile application development, particularly with Ionic but also with other mobile frameworks and technologies. Thanks for visiting.