I did a bit of research into the spacesaver for a mini. It said that on some cooper models its advisable to only use a spacesaver on the rear . If necessary you may need to do a double wheel change ,swopping a full sized wheel from the rear to the front.

I dont know if this is because coopers have larger front brakes calipers, or just a sensible precaution in view of their extra performance.
When I was looking for a second set of wheels back in 2021 I noticed that a major manufacturer of after market steel rims had a version specified for mk1,2 and 3 Jazz but had introduced a new wheel model for the mk4 (which was also suitable for earlier cars) It had the same centre bore size, PCD and E/T off set. I assumed its shape had been changed to allow more clearance for bigger brake callipers. This put me off buying used rims from a mk3, although IIRC some on this forum said mk3 wheels will fit a mk4.
This doesnt fill me with confidence that even a spacesaver that came from an earlier Jazz will be compatible on a mk4 , at least without a test fitting.