Backporting Struts 2 to JDK1.4

I am using Struts 2 and want to run Struts 2 on JDK 1.4. When I searched thru the Struts 2 site it shows me JDK 1.4 jar files to download. But even after using those jar files my application is not working on JDK 1.4. I am getting invalid class version errors. I am not sure what is the problem, but looks like some jars are still compiled in JDK 1.5. I did not find much help on Struts 2 site on this, any help whould be appreciated.