Sony has had a rough going over the past few years. During the 1980s and 90s, it had Apple's current stature dominating huge swaths of the electronics market with its Walkman music players, Trinitron TV's, PlayStation game platforms, and Vaio PC's. Around the word, Sony loss a lot market share to US and Korean players as electronics moved from analog to digital and competition was much more based on integrating hardware, software, and cloud services.
But back in Japan, Sony still enjoys a huge loyal fan base that still want to buy Sony no matter what product category. This is true especially in the smartphone market. While Apple iPhones are strong, Sony leads the Japan market for Android phones. In fact, Sony has totally embraced Android in both their smartphones as well as their smart tv's. Combining Sony's hardware expertise and strong gadget brand with Google's software and cloud dominance is a win-win for both companies.
The Xperia Z4 is Sony's flagship product sold only in Japan. In other countries it's known as the Z3+. The difference between the Z3 and Z4 isn't so great, so perhaps Sony didn't want to set expectations to high by calling it a Z3+. In Japan, Sony needed to keep up their model version numbers close to Apple/Samsung who are already at 6, allowing them to announce the Xperia Z5.
Unlike the high end phones you get from other companies that come with fancy boxes that try to make you feel better about how much you paid, the Xperia Z4 box is very simple and plain. The packaging feels environmentally friendly and doesn't come packed with accessories. Compared to the ostentatious boxes of other phones, opening the box has a very simple zen like feel- it's just a box that's going to be thrown away.
Unlike other companies like Samsung, Xiaomi, and Amazon that use Android but hide or significantly change it, Sony has gone the other away, embracing Android and marketing that as a key feature. For any Android users that want to take advantage of great Sony hardware, the Xperia Z4 is a good choice.