Primary key growth vs. size of table dilema

We have a data warehouse, current version is 11.2 0.3 RAC with partitioning on RHEL 5.10.  This warehouse contains over 30 years of data partitioned monthly with over 100 tables.  We use a 3 or 4 concatenated column primary key on most tables to ensure uniqueness during the monthly loads.  With partition pruning, the primary key (global) is never used during queries.  My problem is I have a number of tables that may have a limited number of columns but require a 4-column primary key.  The end result, as an example, is a table that is 56GB in size with