Description
The bridge, 62 m long and 5.5 m wide, has two large arches (about 24.5 m width) and a central pier with a small hole to reduce the load of the flood water. The inner nucleus consists of tufa and peperino, the external structure is of travertine blokcs, while the covering is a brickwork.
Near the head of the bridge, located on the modern parapets, there are two four-faces marble hermas, in whose lateral grooves the original bronze balustrades were probably inserted.
On the bridge arches it is engraved in block letters four times (two in the upstream side and two in the downstream one), the memorial inscription of the erection dated 62 b.C., while on arch nearest to the bank (and on the corresponding downstream side) a smaller inscription dated 21 b.C. records the authors of a subsequent restoration of the damages due to the river flood of the year 23 b.C. This is the text of the two inscriptions:
L.FABRICIUS C. F. CUR(ATOR) VIAR(UM) FACIUNDUM COERAVIT EIDEMQUE PROBAVEIT
[Lucio Fabrizio, descendant of Caio, Responsible of the Roads, supervised the execution of the job (lit. "the things that had to be made") and approved (them) too]
M. LOLLIUS M. F. Q. LEPIDUS M. FORMER F. CO(N)S(ULES) S(ENATUS) C(ONSULTO) PROBAVERUN(T)
[consuls Marco Lollio, Marco descendant, and Quinto Lepido, Manlio descendant, approved (the bridge) for ordinance of the Senate]

Legends and curiosities
from Giggi Zanazzo on 1907 [3] - pages 284-285 [the original text is in Roman dialect]:
Four Heads bridge.
As all of you know, Sisto V [the Pope], that reigned for five years, get made five roads, five funtains, five spires, five bridges, and left five millions inside the Castle [Castel S.Angelo].
One among the bridges he get restored there was the so called Four Heads bridge.
And do you want to know why it has that name?
Because they told that the Pope get restored that bridge which was falling, by four expert architects, that, while working, quarrelled one against the other so that just for a louse hair [just for a very litte] they avoid to kill one of them.
As somebody informed Sisto V, that, as you know, had forthrights ways, he get catched all of the four architects and in a while he get cut their heads on the bridge itself, and get them exhibit in that place.
Then, for order of the Pope himself, that four heads will made of stone, and they was located, so carved, at the head of the bridge where they still are and gave the bridge its name of Four Heads.

One more interpretation, by Luciano Zeppegno [8] - page 865:
...I might also agree that the four heads are not the four-faces Hermas on the bridge, considering that, in this case, they would be, as properly declared by Delli, nothing less than eight. But I insist on the reason already expressed...: the bridge, being double, that is to say two well distinct bridges, but located on the same axis, it comes to have obviously four heads, that is two on the island, one more on the right bank, and the last one on the left side.
Why to think out other lucubrations when there is a so easy explanation?