Gtx 860m 2gb has higher clock speed and it is more efficient in 8/10 test.
There are two benchmark tests where the 4gb is faster by 2-3fps but thats just because of the memory. The 4gb is sligtly underclocked.
Check maxwell and kepler(if i spelled it right)
Gtx 860m benchmark.
Gtx 880 is far...