You must learn html4 to learn html5, as html5 is built upon an html4 base. However, there are two sources of information on html5 that I've been using so far. One is the w3schools site:
http://www.w3schools.com/html5/default.asp
And the other is the html5doctor:
http://html5doctor.com/
There are lots of others. It's an ongoing process of learning. Watch forums for discussions. Lots of differing opinions on how it should be done!
And, of course, keep watching the specification as it develops:
http://www.w3.org/TR/html5/