Buy if Blackout is Your Battle Royale of Choice
The Call of Duty: Black Ops 4 Blackout game mode is brand new. It delivers a Battle Royale experience with Call of Duty mechanics and weapons. After a month in the wild and several tweaks, you have a very good idea of what this looks like.
It's definitely a different pace and type of Battle Royale than Fortnite and PUBG. You should try it out at a friend's house or check it out on Twitch and YouTube to see if it looks like the type of play you want.
Keep in mind the PS4 PUBG release date arrives in early December, which may be something you want to play instead of spending $60 on Call of Duty: Black Ops 4 for the PS4 to play Blackout, but again it is a very different game.
If PUBG and Fortnite appeal to you, but you want to play Call of Duty this might be right for you. You may still want to wait and see what the Black Ops Pass brings to Blackout.