Required or applicable? It would be required if applicable and called by a regulation.
Regarding applicability, It depends on whether your system provides no basic safety/essential performance or poses no risk. If so, the requirements for PESS would not be applicable as defined in IEC 60601-1 or IEC 60601-1 third edition.
This is not very clear on IEC 60601-1-4, but it´s clear on IEC 60601-1 third edition (60601-1-4 was incorporated into third edition).
Anyway, even if IEC 60601-1-4 or IEC 60601-1 3 ed are not applicable, IEC 62304 would still be applicable (class A medical device software).