three letters. historicly we could get into all sorts of deep phylosopical discussions. supposedly the longer form is about focusing more attention on the person being greeted. but i ask you, is this automaticly or neccessarily a good thing? or even a harmless one? and wouldn't that depend entirely upon the nature of your own thoughts it was distracting you from? =^^= .../\...
hello is what you say as angrily as possible when telemarketers call. hi is what you say to people you like.
yeah hi is definitely more casual, and hello is more formal. and then theres always "hey", which is i think even more casual than hi.