Apple certainly went for a "bigger is better" feel with the iPhone 6, and took it another step with the iPhone 6 Plus. There is a rumor now going around that Apple could be working on a phone that has a 4-inch screen, and this could be the iPhone 7.
According to G for Games, this new rumor originates from Apple's supply chain. The 2015 iPhone line-up should feature a 4-inch model, and the reason is to facilitate single-handed operation with "female users" being an important part of its intended market segment.
I can really see why Apple is making this move. Phone Arena showed a dated commercial for the iPhone 5 that illustrated that the iPhone 5 is made for the distance between your palm and your thumb. They then conclude that it is just common sense. The point is that the iPhone 5 is perfect for one-handed operation, as it had a screen size of 4 inches.
The iPhone 6 has a screen size of 4.7 inches, which is more than a slight increase. Compare the iPhone 5 to the iPhone 6 Plus, and it is a whopping increase as the iPhone 6 Plus is 5.5 inches. Considering that many phones such as the Galaxy Note series are going the phablet (mash-up of phone and tablet) route, it is interesting to see Apple follow this trend.
It is possible that perhaps the 4-inch iPhone planned for 2015 is just one of several models of the iPhone 7, assuming that Apple will even call it that. Apple is notorious for not following a proper numerical order on products, as well as putting out multiple versions of the same products. After all, the iPhone 6 had the 6 Plus, as well as the iPhone 5, iPhone 5S, and the iPhone 5C the previous year. By the way, there is a rumor that Apple could be discontinuing the 5C.
So the new 4-inch iPhone could be one of a few phones that the company puts out. Considering that iPhone accessories are a big market with Otterbox, Trident, and other similar companies who produce cases. Apple's constant changing of their phones increases the market for these accessories, and it is possible that a 4-inch Apple iPhone could be a very easy transition as these companies could just re-use old iPhone 5 cases.
Considering that Apple announced the iPhone 6 last September, it will be a while before the newest version of the iPhone will appear in 2015. A 4-inch model could be on the horizon.