Concept 2 -- "5-4=1"

Here is a second possible concept --

Four characters try to write a screenplay about five people. Each character writing the screenplay is responsible for the dialogue of one person in their script. They share responsibility for writing the lines of the fifth person in their script. The process reveals information about who the fifth character represents to them.

E.G.
The Fifth character is a woman they are all secretly in love with.

The Fifth character is a person they knew whose death they witnessed.

The Fifth character is an amalgam of their own unrealized wishes for themselves.