The "4 Ages of Man" Folio

[MarcoP]

bi3mw,
I really like this comparison! The pose and object are very similar indeed, and the similarity even applies to the curls of hair emerging from under the "physician's" headgear.
ETA: this makes me wonder if, rather than four ages of man or seasons, this folio might depict four professions or fields of study.

One could also speculate that the four figures represent four branches of the medical science, or the portraits of four physicians, like those that can be seen in several medieval manuscripts.
The attached image is from the You are not allowed to view links. Register or Login to view. (1441).

bi3mw's parallel is extremely interesting, still the context of the illustration and the structure of the diagram are suggestive of a cosmological subject. The four elements (or possibly the humors or the ages of man) seem more likely to me.