Water Pump Difference between 4th gen & 3rd gen ?

Maybe different electronics, ie: it'll bolt on, but the computers won't know what to make of it.

 

Stay in your generation and only use oem Aisin pumps. Aftermarkets often fail quickly while examples like my car's oem pumps are about to cross 300,000 miles. Changed coolant about ten times. An overheated aluminum engine will ruin your day and may end the engine’s life.

 

Here is a relevant video just published today

 

Is it just missing thermal paste, or there’s more, and how would you know?

pretty sure the “non-original replacement” on this page is the one GM’s are giving the thumbs up:

https://www.amayama.com/en/part/toyota/161a039015

just for giggles would check for thermal paste upon receipt.

 

There is thermal paste, and there are other thermal interface materials (TIMs), such as thermal conductive pads. Various things are assembled with them. Some of them are adhesive, and applied just the right way at the factory. In the video, those looked more pad-like than paste-like to me.

It can be tempting to take one apart and look ... but with some of those materials, assembled in ways similar to how that pump looked, once you pull it apart ... you've just signed its eventual death warrant from overheating once reassembled, because the TIM won't be doing its original job.