I can assure you that the hardware is completely identical because I flashed the R3GV2's firmware on the 4AGE using a ch341a flash programmer. The device worked fine for the last 2 years but recently I broke openwrt when upgrading and that to run the tftp-restore-method: i did not work as the 4A's restore-image was not accepted.
I had to use the ch341a again to flash it and this time I flashed the 4a-image containing the bootloader, Bdata and so on: it was a perfectly fine working 4a again. This way you can also convert a chinese device to a EU/DE-device and the other way round.
Now comes the interesting part: you can interchange the bootloader by flashing mtd from a running openwrt after installing the kmod-mtd-rw and then using "mtd write file.bin partitionname".
This way I converted my second 4AGE running a R3Gv2-Image back to the 4A's bootloader.