Call of Duty: Black Ops 4 will require under 50 GB of hard disk space for the initial install, and you can also preload it ahead of launch.
Treyarch announced the preload date for the PC version earlier this week where they confirmed that the preload for the PC version will go up on October 9th. As for the PS4 and Xbox One version, they are also expected to get the preload 2 or 3 days before launch, so you should be able to download the game before it becomes available on October 12th.
As for the file size, while the back of the cover lists 100 GB as the required hard disk space, the official PlayStation Store and Xbox Live Store have listed the file size as 42 GB to 48 GB respectively. This is likely without taking into consideration a possible day 1 patch that will be available closer to launch, so the size will be higher once the day 1 patch is factored into it.
Call of Duty: Black Ops 4 is the first game in the series to launch with a proper main story campaign, however, the focus has been on the Blackout mode that promises 100 player battle royale for the first time in a Call of Duty game.
Call of Duty: Black Ops 4 will be available on October 12th for the PS4, Xbox One, and PC.