I've read the tutorial: http://www.millisecond.com/support/docs/v3/html/howto/multiplescripts.htm

According, to the tutorial:

"The batch file is very easy to use, but it does have some limitations. Most notably,
there is currently no built in way to randomize the order in which the scripts are
run (this is true as of version 3.0.2.0, but the feature is planned for a future
release)."

Is this feature available in Inquisit 4? If so, how do I write the script?

It is already available in Inquisit 3. See the documentation for the <batch> element's /subjects and /groupassignment attributes.