speak volums vs say vs are proof

How would you say this sentence? Could you tell me if these are all natural and which you would maybe say?

The people who work for him speak to how bad he is at hiring people.
The people who work for him speak volumes about his recruitment process.
The people who work for him say a lot about his recruitment process.
The people who work for him are proof of how poor his judgment is in hiring people.

Thank you so much