FWIW, my older Sam's Tube Substitution Handbook lists it as two-way interchangeable (meaning either can be substituted for the other) with all other versions of the 5Z4, as well as the 5CG4. The 5V4GA, 5V4GY, and 5AR4 are listed as substitutes for it, but not the other way around.
Regards,
-- Al
Regards,
-- Al