Can my PC run modern games with 4GB of RAM? [duplicate]

Short Answer:

No, you need more RAM based on the minimum system requirements of the games. The games minimally require 6-8GB of RAM.

Long Answer:

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.

The RAM specs for those games are as follows (for 1920x1080 resolution):

Game          Minimum     Recommended
GTAV          4GB         8GB
ME:Andromeda  8GB         16GB
Witcher 3     6GB         8GB

For modern games and to keep your PC reasonably future-proof you'll want to look at the higher end - 16GB at least, maybe even 32 if you can manage.