I have heard the PS4 has recently been updated. I like some of the changes, including the mechanical power and eject buttons, but I especially like the improved power supply. I read some articles about the updates, but I have yet to read a list of which special editions feature the upgrades. Which special editions of the PS4 feature the updated power supply and other upgrades?
The new version of the PS4 is called "CUH-1200". There are different numbers for this version, depending on the territory, e.g. the European relese is called "CUH-1216", the US release "CUH 1215".
There is also a letter after this number, signifying the HDD space. The 500 GB version is called "CUH-1216A" and the 1 TB version "CUH-1216B". I'm not sure about the exact release date, but the 1200 models have been released somewhere around July/August 2015, so any console from before that time will be one of the older models.
Make sure to look up the number before purchasing the console. It should state it somewhere on the package, otherwise you should ask the seller before purchasing. It is also a good idea to search amazon reviews for clarification which number the console has, if it isn't stated in the description.
In some countries, online stores will show the version number on the article page, like amazon.de.
In case you're interested, here is a good comparison between the three models.