ISX04 - What is the purpose?
Section 5 of the document "The ISX Segment in Operation" at https://isx.x12.org addresses this RFI.
The purpose of ISX04 is to indicate that only the codes listed within a referenced implementation guide are usable. For example, data element 128, Reference Identification Qualifier has over 1,900 code values. Implementation guides usually don't allow the entire list and publish a subset. For this purpose, that subset is identified by content in ISX04. If a code not explicitly listed in the implementation is sent, that is an error condition when ISX04 is used.