Do we actually know that to be the case though? Jiraiya and the toads didn't know there were other Pain's at this point, so they would've just stopped the Genjutsu after killing the first three.
Also, the Pain's share eyesight and the toad's Genjutsu is auditory whereas Itachi's is visual.
Also, the Pain's share eyesight and the toad's Genjutsu is auditory whereas Itachi's is visual.
this would imply that each Pein is counted as a distinct entity otherwise Nagato himself would be there for hearing it.