Could I offer an alternative suggestion to this thread I know this doesn't give you rear speakers but some people may just want improve their overall sound quality. I have replaced the front speakers in both my MGs (ZS/MG5), using the same units transferred, JBL GX602 165mm and I also fitted a Pioneer Under seat Sub unit TS-WX130 135w with built in class D amplifier. I don't know if you have enough space under the MG4 seats the MG5 is very tight but the wiring kit has plenty of length so I have fitted the sub in the boot under floor. The difference is great and to me is well worth the money as I am music mad. The increase in bass and overall sound is substantial. You will need 'car speaker spacers/adapters' they are about £7/8, the JBLs are £45, the sub with remote wired controller unit £145. There are some JBLs that come with separate tweeters which could be used to replace the existing ones see amazon links below.
Amazon product ASIN B00MUKMYNY JBL GX602 series.
Amazon product ASIN B078PHCHSW Pioneer TS- WX130DA