Running 4 different user accounts separately

I am currently trying to see if it is possible to create 3 different user accounts that cannot interact with each other and additional one that can interact with 2 of them on the same website. They are all have different maturity levels and they need to be separate for that matter. For example, 1,2,3 can not speak to each other but 4 can only talk to 2,3. What process or plugin will allow me to do this on my Website? Thank you so much in advance for the responses!!