We have a group of 3 marketing users that enter, edit and use in CRM.  We want to add another user with their own set of contacts, but need these to be read-only to the 3 marketing users, and this new user can edit, delete, enter contacts that he owns.  The new user's contacts need to be accessible in marketing campaigns that the 3 users create.

If I created a new business unit, this would fix the second issue, but then the existing 3 users cannot see these contacts.  I can setup the security role to allow Read and write contacts, but if all 4 users are assigned to the same role, anyone can edit this new user's contacts.

Is there a way to share ALL contacts, yet segregate that 3 users can edit, write some contacts and another contact could edit the ones he owns?

We do not use the "sharing" contacts as 1) you can only share 250 contacts at a time and 2) would need to share everytime you create a new contact and users always forget.