This kind of start screen layout (4 medium tiles per row, less space between tiles) can be seen in a number of demos and slides from build. Seems like they want to make better use of the screen real estate on phones. I guess that this specific layout will be available on larger phones only, but maybe the smaller ones will have the narrow spaces between tiles as well (but only 3 medium tiles per row). I wonder if the desktop version of Windows 10 will have that as well, so far we haven't seen this new layout on the desktop. Considering that Belfiore described the demoed start menu on desktop as near-final, I don't think it will change much.
Keep in mind that the start screen on phones is probably still more of a work in progress than the desktop start menu. For example, the tile grouping feature that is available on the desktop is not implemented yet (it was included in all of the official mockups we have seen so far).