I was thinking along the Same lines I kept my MG5 genuine MG spare wheel and tool kit which is 16inch knowing that the MG4 wheels are 17inch so I have not tried to fit it as yet but when I placed it against the wheel on the 4 there is only about half an inch difference between them, now this perhaps is because of the tyre profile so if the studs holes or the same I think it might well be fine.
The only issue would be unlike the 5 the 4 as no spare wheel well so would have to live in the boot.
This is something that as confused me for years as when we purchased the wife’s Corsa
Brand new in 2005 which we have just sold with just over 60,000 on the clock and as good as new to purchase the MG4 for her the Corsa had 15 inch wheels but the space saver spare that came with the car from new
( yes in them days cars came with a spare wheel and a jack and tools ) and the spare that came with the car was 14inch not that we every had to use it.
Les