gtx 860m one too many for me to understand

Okay so there are 2 versions of the gtx 860m, gk104 and the maxwell one gm107. So according to the reviews the gk104 one has 4gb while the gm107 has 2gb but according to lenovo their y50 is using a gm107 with 4gb. First question - is this possible and 2nd question - just how much of a difference is there between the gk104 and gm107? Thanks!