Saxon

The class ValueEntry4 is used to hold key values when there are exactly four fields in the constraint. The equals() method for ValueEntry4 says:

        public boolean equals(Object obj) {
            return obj instanceof ValueEntry3 &&
                    compareValues(value0, ((ValueEntry4) obj).value0) &&
                    compareValues(value1, ((ValueEntry4) obj).value1) &&
                    compareValues(value1, ((ValueEntry4) obj).value2) &&
                    compareValues(value2, ((ValueEntry4) obj).value3);
        }

The first condition (obj instanceof ValueEntry3) will never be true, so the keys will never be equal, so xs:unique and xs:key will fail to report duplicates. Conversely, an xs:keyRef constraint with exactly four fields will never be satisfied.

(Spotted in the code; no test case has actually failed.)