There isn't any one thing about JDK 1.4 that constitutes an advantage; most of the changes are just new features that are very useful. That said, I would say that the New I/O (nio) system, in the java.nio.* packages, are probably the most significant change. They are a completely new way of doing I/O, although they are well integrated with the original (java.io.*) method.
<a href="http://www.panix.com/~mito/" target="_blank" rel="nofollow">Greg Travis</a>, author of <a href="http://www.manning.com/travis/index.html" target="_blank" rel="nofollow">JDK 1.4 Tutorial</a> from <a href="http://manning.com/" target="_blank" rel="nofollow">Manning Publications</a>.