1.4 is version 1 and 2.4 is the version 2
Basically they just built up the library with a lot more options esp with schema if I remember right.
Here is a link to show you the features and other things of version 2
http://xml.apache.org/xerces2-j/ and here is the same thing for version 1
http://xml.apache.org/xerces-j/ Eric