I don't know if this helps your case, but for me, running OSX 10.5.8, I had to go to Java Preferences and swap out what version to use for Java Applets from J2SE 5.0 to Java SE 6.
I used this page (http://skyreacher.net/?p=90) to help me change out the version I'm using.
After I did this swap, the .jar worked! Again, I hope this helps, though I'm not sure if OSX 10.4 will have this built in.