Yes
offcourse, you can create 1:N, N:1, N:N relationship with full support from MS CRM 4
But relationship role. i don't think so. i haven't seeen any example of it.
but NO for relationship role is only restricted to contact or account and as per sdk
relationshiprole Class (CrmService)
Represents how an account or contact affects, influences, or contributes to an account or contact in a customer relationship.
For a list of methods and messages that operate on this class, see Relationship Role Entity Capabilities.
The relationshiprole class inherits from BusinessEntity. It exposes the following members
This entity is not customizable.
Muhammad Ali Khan
My MS CRM blog