No, you need more RAM based on the minimum system requirements of the games. The games minimally require 6-8GB of RAM.
Without really looking too deeply into the minimum system requirements I would personally recommend upgrading your ram. 4GB of RAM is on the verge of being useless in a gaming computer.
Looking into the minimum system requirements of Witcher 3 and assassin's creed Origins, both require at least 8GB of RAM.
As per what settings you would be able to play the games on all depend on the graphics card and the rest of your system. I would just go with whatever the recommended settings are when you launch the game.