From Apple’s developer blog:

The size limit of an app package submitted through iTunes Connect has increased from 2 GB to 4 GB, so you can include more media in your submission and provide a more complete, rich user experience upon installation. Please keep in mind that this change does not affect the cellular network delivery size limit of 100 MB.

Over the years, the maximum size of 2 GB for apps forced game developers to drastically reduce the graphical capabiltities of their games (see, for instance, the original BioShock port). With Metal and more powerful iPhones and iPads, it only makes sense to let developers create more advanced iOS gaming experiences with fewer compromises.