If the confluence was that of the ARDA with the Po, Tacitus' quartum is still unsatisfactory, but the distances given in Plutarch's Life of Otho would suit the facts.
"Tacitus: The Histories, Volumes I and II"
Caius Cornelius Tacitus
From the camp then pitched to the mouth of the ARDA would be by road about sixteen miles.
"Tacitus: The Histories, Volumes I and II"
Caius Cornelius Tacitus