This is covered in ¶ 365. EP IV has an invariable preface and so is not permitted on days with proper prefaces; other than that, there are suggested usages but no requirements.
I rather like EPIV but I’ve heard it only once in all my life (that I recall). It was on All Souls Day when the priest used the Preface for the Dead then then inserted the Proper Preface for EPIV after the Sanctus. Very odd.
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