The Viber service is only available between 2 Viber users at this stage. Meaning you could use Viber between anyone who has an iPhone, Android, iPod or an iPad (and soon also Blackberry and Nokia devices).
Viber already works on iTouch 4
Install it from AppStore, and try it out.
When asked for a number for registration, you'll need a different number than the one you registered on your iPhone (otherwise it will automatically deactivate the account you already have on your iPhone). You could use your landline number in order to receive the code, an old cellphone number, a family member's number, etc.