menu Edit->General Preferences, tab Render. Set the number of threads to 4.

You can also set Poser to render in a separate process. This will enable you to render more complex scenes, since Poser has a limit of 4 GB per process, and the separate render process won't have the overhead of a user interface.
Renders will be slightly slower than rendering in the Poser process itself, since inter-process communication takes some time.
Unlike what some other poster has stated, if you do not render in a separate process, Poser will still use as many cores as you have set the threads to.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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