Is ORDER BY on two indexed columns will be slow?
If ID is the clustered index, on SQL Server this index will have ID added as the last index key.

For a non-unique non-clustered index on a clustered index table the clustered index keys that aren't explicitly listed as non-clustered index keys will be added as additional non-clustered index keys. And the non-clustered index will actually be stored as a unique index.  EG your index on (a,b,c,d) is implemented as a unique index on (a,b,c,d,id).  

If the non-clustered index is unique, the clustered index keys will be present on the leaf level of the index, but will not be index keys.