There is not limit for the amount of users you can create on a My Cloud EX4, nor a limit for the people that can access at the same time. Depending on how many users access at the same time, it will probably slow down the user interface slightly.
Same thing for the PR4100
You can use the HDD standard. But try for Black edition. The reason NAS certified is it meant to run for 24x7 like you can also try surveillance series from any manufacturer. Try NAS or Surveillance or Black edition.