In following document, it illustrates how user can use jre 1.4.2 plugin when he also has jre 5.0 or 6 installed in his system.
http://java.sun.com/javase/6/docs/technotes/guides/deployment/deployment-guide/jcp.html#java

However, I tried this procedure on several configs and found that in all cases, version 1.4.2_15 Plug-in didn't run in IE. Either jre 5.0 or 6 was used to launch applet.

XP SP2, IE6, jre1.5.0_15
XP SP2, IE6, jre1.6.0_03
Vista, IE7, jre1.5.0_15
Vista, IE7, jre1.6.0_03

All windows tested are Japanese versions.
IE6 version: 6.0.2900.2180.xpsp_sp2_gdr.070227-2254
IE7 version: 7.0.6000.16386
I fixed this issue by adding a note in the jcp.html as below.

Above mentioned example only applies to pre-1.5.0_06 (pre-SSV) versions of the plugin. See the changes in 1.5.0_06 version (http://java.sun.com/j2se/1.5.0/ReleaseNotes.html#150_06). However, the new Java Plug-in technology introduced in JDK 6u10 would enable the user to choose the version of JRE, using different version selection mechanisms, as described in the New Java Plug-in release notes (http://java.sun.com/javase/6/webnotes/6u10/plugin2/version-selection/index.html#MECHANISMS).

This should be online in jsc/javase/6/docs website within 24 hrs.
- Jacob Royal