Assuming you want to play Fallout 4 at 1080p a GTX 960m would suffice.
If you don't mind playing at lower resolution less than high graphic settings then playing Fallout 4 with a weaker GPU is possible. For example, I played Fallout 4 on my laptop with a Radeon HD 8850m @ 1600x900 resolution and a mix of low and medium settings. It is a little more powerful than a nVidia 940m.
Do not buy a laptop with integrated graphics to play Fallout 4 like Intel HD graphics because they are not powerful enough to play Fallout 4 and they are not officially supported by the game. There are some fixes to get Fallout 4 to run on Intel HD graphics, but it is hit or miss. Some people can play FO4 using Intel HD graphics (low performance) while others...