You can possibly do this:
Download the latest version of JDK 1.5. With it, run the compiler with the -target 1.4 argument to javac. This will generate
Java 1.4 compatible class files. Whether this will work with your java 1.5 class files I don't know, but it will at least compile the source.
Honestly? Either you have to find a Java 1.4 compliant version of whatever libraries you're using, or you need to upgrade your project to Java 1.5
Jeremy