http://www.oracle.com/technetwork/java/javase/index-jsp-138567.html
Not only is Java 1.4 not supported, Java 1.5 (Java 5) reached End of Life April 8, 2008 and Sun/Oracle cease all support November 3, 2009. Use these obsolete versions of Java at your own peril.
Since unlike a certain other popular provider of programming languages, Sun does not abruptly drop people off a cliff when they're tired of an old release of software, you can squeak by using the compatibility-level options on the JVM. However, nothing lasts forever, and the longer you put off upgrading the code, the more it's going to cost.
People like to use the phrase "If it aint Broke, don't fix it!" in IT, but they rarely stop to think that just because "it" ain't broke doesn't mean that resources critical to "it" haven't deteriorated. Software doesn't exist in isolation.