It's never that simple, really. Is the extra person just "an extra pair of hand" or are they filling a spot that nobody is currently successful at?

I'm in a 4 person team currently, give me an additional project manager and productivity will soar as devs can focus on technical aspects, rather than being inefficient at PMing for a mediocre output (at least that's my expectation, you never know in advance I suppose)

Agreed, if people are in different roles, an extra pair of hands can be fully productive. As soon as the fifth person is eg working on the same codebase of the previous four, there arises necessary overhead and stepping on each others' toes.

It would depend upon the work.

If the work is embarrassingly parallel, then 25% more. (5/4 = 1.25, of course)

If there is too much work for four people to function effectively, perhaps more than 25%.

If the work requires a lot of inter-process communication, perhaps less than 25%.

You're right, thank you.