There’s been a lot of discussion of this topic.
I took a shot at fixing the MultiVolume Importer a few years ago and found it impossible to pin down any real standard for 4D nifti, but made things work for the files on the nifti web site. Beyond that I tend to throw up my hands and consider any other files application-specific and not interoperable.
@Vincent_Magnotta if you can make the case that there is a valid definition of 4D nifti that will allow true interoperability then for sure we would want Slicer to read and write those correctly, ideally via ITK. Things may have changed, but nfortunately last time I looked there was no such definition. Even 3D nifti has some issues.