Of þe uour uirtues cardinales spekeþ moche þe yealde philosofes. ["Ayenbite of Inwyt," c.1340]
By analogy of this, and cardinal points, cardinal winds, cardinal signs (four zodiacal signs marking the equinoxes and the solstices), the adjective in Middle English acquired an association with the number four.
Although the work does not treat of the recent means of diagnosis--the thermometer, laryngoscope, cardiograph, etc.,--still it is complete as far as it goes. [book review in "Medical Investigator," May 1867, p.94]