With the Windows 10 preview build 10547 for fast ring Windows Insiders, Microsoft now allows you to add more tiles in a Start menu/Start screen group. In the prior version, it only supports 3 columns of medium sized tiles but with this build 4 columns of medium sized tiles is possible.
Continue reading this post to learn out more on this topic as well as to know the way to change the settings and allow more no. of tiles in a Start group.