HTML 4 became a standard in 1997
HTML 5 might become a standard in 2017 (although it might take longer)
It took about eight years after HTML 4 came out before everyone was using browsers that supported it.
Almost everything in HTML 4 is also in HTML 5 so you need to know HTML 4 in order to use HTML 5.
Some of the things proposed for HTML 5 are only needed to fix IE6 problems and so presumably now that browser is dead those things can be dropped from the proposal (eg. embed). There are also some proposals for HTML 5 that provide more than one way of doing the same thing and so presumably only one will make the final standard eg. the input 'required' attribute provides a subset of what can be specified with the 'pattern' attribute..
Basically HTML 5 is currently available for testing so that people can provide feedback on what works and what doesn't so a decision can be made on which of the new proposals will survive into the standard.