If I delete a game that comes with my PS4, can I reinstall it?

YES,

If you purchased the PS4 and had a Digital Download or Disc to Call of Duty: World War 2, you will be able to re-install it at any time. In the case the game came as a digital download, you can go to the "Library" section of your PS4 and go to "Games" to find the game and re-download it. If you received a disc for the game, you can simply put the disc in the disc drive and re-install. Your saved game data should still be on the PS4 if you did not delete that data since the last time playing.

In the case that you don't have it digitally in your Library and do not have the disc anymore, you can purchase a "pre-owned" copy of it from your local game establishment and then use that disc to re-install and continue from where you left off in the game.

I hope this helps, Cheers!