I'm guessing you need to buy both to get Horizons on the PS4, is that correct?
Thanks.
The base game is part of Horizons. You can't have one without the other.
You do have the option to buy just the base game without the ability to land on planets etc. but this will lock you out of 75% of the in-game activities, so I would not recommend that as a viable choice.
All of the latest alien activity for example is playing out on the surface of planets, so those who don't own Horizons are going to be unable to participate in any of that gameplay directly. Same goes for a large chunk of missions including the highly lucrative passenger transport missions.
The dollar difference between the two versions is hardly worth it to be losing out on all that stuff.