Full Idea
To the four Aristotelian causes Plato adds a fifth in the model - what he himself calls the 'idea' - this being what the sculptor had constantly before his eyes as he executed the intended work.
Gist of Idea
To the four causes Plato adds a fifth, the idea which guided the event
Source
Seneca the Younger (Letters from a Stoic [c.60], 065)
Book Reference
Seneca: 'Letters from a Stoic (Selections)', ed/tr. Campbell,Robin [Penguin 1969], p.119
A Reaction
A very interesting interpretation. I take the four 'causes' to be primarily the four 'explanations', and it exactly fits how we should understand Plato, as offer a crucial underlying explanation. The statue is Aristotle's example.