@dkw1 

Hey

This has been asked a number of times for other hardware, not specifically Deco.    In short, the answer is the chip shortage, as the chips TP Link use in the v2 model are in short supply / not possible to get at present, they have had to switch chip supplier and that resulted in the v3 and v4 being released.

Fundamentally they are the exact same specification and features, with exception of the chipset being from a different supplier. 

FYI.. Usually a new model (v2 v3 etc) would be an upgrade over the previous model (v1 to v2 was for example) however given the current situation in the industry version changes are purely reactionary based on supply, there is no reason to suspect that v3 or v4 is better than v2, it just uses different components to achieve the same result.