That's the official cover, but as with anything like this it's subject change.
Although similar it is worth noting that the colours in the Amazon image are incredibly saturated, making it appear similar to Season Three. But if you search a better picture of it on Google and watch the trailer you'll see it's a much paler turquoise and less green. The other obvious differences are it features Guinan and Riker and Picard in a different pose to the previous seasons. So I don't see any reason to get confused, you only needs to look at the title listing to be able to tell which season is which, let alone a cover.
It no doubt will, as all the other sets have, including Enterprise.
Bear in mind that the image on Amazon is just there to represent the product and not depict what it'll exactly look like. All the other images of the Season Four set show it with a slipcover, so I have no concern about a lack of slipcover
The released box has the same image as shown on the Amazon page in the UK, with the exception of the BBFC rating 12 in the bottom left of the box which is not shown on the Amazon image (at least when I posted this)