@Arion 

On the note for firmware upgrade, the v4.20 is only available for 4.20, and v4.0 only available for 4.0. Also you can see that the file size is different. I think that they are not compatible.

I still need an official answer from TP-Link staff.

Sometimes developers say that decimal version difference is not version difference. Even in this case, firmware update for 4.26 is written officially the same file as for 4.20

The website suggests that the firmware should be compatible, specifically, on the v4 firmware page, it says Normally V1.x=V1.

Also, for How to find the hardware version on a TP-Link device under the heading "For products with management interface:" it says "There is a character string “Ver:X.Y” (for example, Ver:1.0) in the Serial Number field, and the number X is the hardware version of the device. If the string is “Ver: 1.1”, normally it means that the hardware version is V1."

The v4 firmware on the website is 437.51 KB while the v4.2 and v4.26 each have two updates of 438.09 KB and 437.51 KB. It's not clear if the slight size difference is significant.

However, the v4.2 firmware update says it's for hardware version 4.2, and the v4.26 update says it's also for hardware v4.2.

If the v4.2 hardware is incompatible with v4, they should have just bumped it up to v5 for clarity. I also don't understand why both 4.2 and 4.26 appear as distinct entries in the drop-down list.

What a mess.

The last time I tried a firmware update on a TP-Link device, it bricked, so I guess I'm not doing anything until this versioning issue is explained.