Is GTX 860M 2GB enough for gaming at 1080p?

I'm planning to buy the Lenovo Y50 this fall, especially for playing GTA V. Is the 2 gb version enough for 1080p gaming? Or should I go with the 4 gb version?
Will the 2gb version be enough for low-med gaming in next 4-5 years?

Yes it should be enough.

It's not enough for 1080p. Most mobile video game cards are very weak at rendering modern games at FullHD. Even if you get the 4 GB version, this won't change the fact that it's an M (mobile) card. Prepare to set the graphic options of most games to Medium/720p, or get a dedicated desktop PC.