The GTX 960m is overall pretty good to play games at 1080p. Hardcore gamers would likely consider the GTX 960m to be the "weakest nVidia GPU" gamers should consider when looking for a good gaming laptop.
While the GTX 960m can play many games at 1080p with pretty good performance with high / ultra graphic settings (40+ FPS), there are some games where you need to drop resolution and or graphic settings to achieve 40+ FPS.
For playing games at 4k resolution... the GTX 960m is too weak for that regardless if it had 2GB, 4GB, 8GB of DDR5 dedicated video RAM. To play games at 4k resolution, the laptop needs to have at least a GTX 980m, GTX 970m SLI, or a tweaked desktop GTX 980 GPU in it.
See link below for GTX 960m benchmarks...