1.) You can find all downloads at http://java.sun.com/j2se/downloads/
2.) This is not your problem, because you can also code 1.4 applications with 1.5.
You probably have a class version error. Post the complete error message with stack trace. You probably have some code compiled for one version and some for the other version.
his problem is that he's trying to execute a class (or set thereof) compiled with a 1.5 compiler on a 1.4 runtime when he's not taken care to tell the 1.5 compiler that such is his intent by supplying the correct commandline arguments to said compiler.
Upgrading the client workstations to a 5.0 runtime would be the best solution overall, if that's not possible the classes will need to be recompiled (and possibly rewritten) for the 1.4 platform.