I don´t think so..... WiiU is definitely a 8th generation device.

A videogame generation is related to the release and availability in the market of any given group of devices and not to the power of the device itself. Or we wouldn´t say, for example, that Neo Geo is a 4th generation console, because it was more powerful than the most popular devices at the time. That would be wrong, because its launch and availability fits the same period of SNES/Genesis.

Personally, I think 4-year cycles are too short. Shorter cycles and the rush to go to the next piece of hardware means more expenses for the costumer and underused hardware. I think 6-7 years  would be ideal

Wii U is not a mid gen upgrade, but it funny that some people used to think that because the lack of advertisement the Wii U got.

Does the Wii U qualify as a mid gen upgrade?

No

or the DS/3ds iterations?

Yes

Why the blow back on PS4/k?

It costs more than a handheld