I currently have the 15" Retina Macbook Pro and when I use any one of the thunderbolt ports to my external monitor my MacBook switches to the dedicated GPU. However the new 13" Retina Macbook Pro has the same dual thunderbolt ports so it will be interesting to see how the HD4000 can handle dual displays. As a side note the Google Chrome browser at times will kick in the dedicated GPU on my 15" Retina Macbook Pro. Now granted the display on the 13" is a little less in terms of resolution compared to the 15" but not by a whole lot to make that much of a difference. Anyways just wondering how the HD4000 will stack up in the 13" Retina Macbook Pro.