2nd Solution - I set a master user (or a dedicated inbox) , create a shared folder with all 4 user. Eveybody can read and reply (or write, a brand new name, with other setting) in the name of the "master user" or "dedicated inbox user".
I supposed more or less to have solved the problem, but i found that in this way only the "master user" can put TAG, and all the TAGs are only for his own use, and other delegate user can't see or type a TAG on the email in the shared folder.

I supposed to create a Tag with the name of each user, in a way to have a fast view (and filter) of who do what (to show to other: Guy's I'm working on it!)

well, all the way u try to share completely something, there is some limitation.

I read discussion from users about this topic since 6 years ago.

Do you have any suggestion or new emprovements?

I like Zoho, but this missing feature stop me to migrate.

Thank you