TP-Link partially broke TFTP recovery with the v4. You will need to pad the Factory image, as TFTP Recovery uses wrong offsets.
The C50v4 is indeed hardware-wise completely identical with the C50v3, the difference lies within a TP-Link design-choice. They are using a split-uboot design, already present in some way on newer QCA based devices. It consists of a "factory-uboot" and a "image-uboot". Factory-uboot is never touched and contains recovery-routines (TFTP / HTTP). "image-uboot" seems to be updated with every firmware-release.
I ported OpenWRT on the C50v4, but i'm not sure if my design with integrating a from source pre-compiled uboot image is the right way. There is no way to flash an image without the "image-uboot" as every way (WebUI / TFTP / HTTP) requires this bootloader to be in place.
If anyone is interested, it can be found here (will rebase it to latest maste later this weekend): https://github.com/blocktrron/openwrt/commit/c38577305289fd1d0659d5e1008eda6104869a6a
It will poop out a factory-image (For use with GUI and HTTP recovery) and a tftp recovery for use with the partially-broken TFTP recovery (Hold down reset button as usual). In case you want to go back to TP-Link firmware, note "Modified firmware-uboot" in the linked commit message. Enter this HTTP recovery, there you can upload the TP-Link firmware.
Everything i'm aware of is working fine and we even have support for TP-Links new U-Boot integrated HTTP recovery.