You could create a separate "key master" table, containing only an identity column and the four natural key columns, clustered on the natural key columns, NOT the identity.
Then, the first time a given combination of
A1, B2, C3 and D4
values are to be inserted to the main table, you first insert a row into the "key master" table; then, when inserting to the main table, you lookup the identity key value to be used from the "key master" table.
The main table can also be clustered on the natural key columns, if that works best for the queries, while the PK could still be the single identity value.
SQL DBA,SQL Server MVP('07, '08, '09)
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