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 and Ionic 2 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 2 but also with other mobile frameworks and technologies. Thanks for visiting.