Commentary Par XVII 24

For the word 'tetragon,' see Chiavacci Leonardi (Chia.1989.2), pp. 312-13, and Raffa (Raff.2000.1), pp. 164-78, both of whom consider the two sets of meanings of the geometrical figure that may have influenced Dante's choice of the word here, defensive (it was reckoned by several authorities, including Aristotle and St. Thomas, to be the strongest shape capable of withstanding assault) and more positive (in one medieval tradition it is associated with Christ).

Chiavacci Leonardi (pp. 314-16) also adduces Boethius here, as model in the widest possible sense.  In her view, he, like Dante, persecuted and unjustly condemned, wrote a work of which he, again like Dante, was both author and protagonist.

For the Cacciaguida episode as also reflecting the sixth book of Cicero's De re publica, known as the Somnium Scipionis (and in this form commented on by Macrobius), see Schnapp (Schn.1986.1, e.g., p. 62, but passim) and Raffa (Raff.2000.1), pp. 147-64.  And see Schnapp (Schn.1991.2), p. 216, discussing the similarities and differences between the prophecy offered by Brunetto in Inferno XV and that by Cacciaguida here.