Preemptive Web Services allow your applications to take full advantage of multi-core computers resulting in faster responses for simultaneous requests. It’s not only powerful, it’s also easy: from the Method Properties of any method, just select the “Can be run in preemptive processes” option and you’re off and running!

A 4D method can be preemptive or not according to compilation options and the use of the language, as explained in this post.