A new, peer-reviewed paper has just been published in The Journal of Analytical Psychology (JoAP), with the title The five functions of psychological type. JoAP is the leading international journal primarily for Jungian Analysts.
The paper argues that it is better to refer to psychological type theory (better known as Myers-Briggs typology) as containing five functions, not four.
The abstract is openly available at the JoAP website. This blog page provides a summary of the content.