Return to Forum of Nerva
"...formerly you lived on a passage in a tiny dwelling, where Rome in her crowds trod the thoroughfare. Now your threshold is encircled by Caesar's gifts, and you number as many forums, Janus, as you have faces."
Martial, Epigrams (X.28)
The Temple to Minerva was situated at the end of a long narrow piazza, within which was a Janus Quadrifrons containing a statue to the god with four faces, each looking out at one of the fora, said to have been brought to Rome after the conquest of the Etruscan town of Falerii in 241 BC. There is no archaeological evidence for the Janus but, given the name, it likely was on an axis with the other forums. One possible location is in the center of the courtyard (as pictured above), although Richardson and Anderson place it near the entrance to the Forum Romanum, opposite the Temple of Minerva.
In the early decades of the third century AD, Alexander Severus "set up colossal statues of the deified emperors, some on foot and nude, others on horseback, with all their titles and with columns of bronze containing lists of their exploits, doing this after the example of Augustus, who erected in his forum marble statues of the most illustrious men, together with the record of their achievements" (Historia Augusta, XXVIII.6).
Much of the temple, itself, which was prostyle, hexastyle, with a projecting pronaos set on a high podium, still was standing early in the seventeenth century when it was pulled down for its marble, having been robbed of its statuary and stone all through the previous century. A marble fragment of a frieze with bucrania show a similarity to the decorative motif of the Temple of Vespasian.
The Janus Quadrifrons sometimes is confused with the Janus Geminus.
Reference: A New Topographical Dictionary of Ancient Rome (1992) by L. Richardson, Jr.