Well, does your "dmesg" show anything relating to FS corruption or like? If you have too many ecc errors on the NAND then the mtd4 partition cannot be used and it will be mounted read-only.

I have been told that some of the NAND chips are quite error prone, and that is the reason Nokia did not use the mtd4 partitions for /var filesystem as was original intent.

There is another way to store the boot manager, using initramfs on a mmcblk0 partition. I will have to explore that as an alternative if many people start getting the write errors on mtd4.