We have 4 document libraries in a site (Doc1, Doc2, Dco3, Doc4). There are around 20 users in the site and out of them for 4 users we want contribute permisions on the Doc2 and read only permission for the rest of the site. Similarly other 4 users we want contribute permissions on the Doc3 and read only permission for the rest of the site. What would be the best way of doing it?
The best approach is to create Sharepoint Groups for these users. Break the permissions inheritance on the desired document libraries (Document Library Settings, Permisions, Actions -> Edit Permissions, click OK) and set the permissions as you want them to.