1.4+ is a mistype, I suppose. According to http://sourceforge.net/p/graphicsmagick/code/ci/tip/tree/ they don't even have 1.4 branch. The latest release is 1.3.18
This extension works best with GraphicsMagick version 1.4+. Gmagick should work with earlier versions up to GraphicsMagick 1.2.6 but some features and formats are not supported for these versions. Windows version of GraphicsMagick is available from the GraphicsMagick website. To read about the difference between ImageMagick and GraphicsMagick, check the » FAQ.
1.4+ is a mistype, I suppose. According to http://sourceforge.net/p/graphicsmagick/code/ci/tip/tree/ they don't even have 1.4 branch. The latest release is 1.3.18