It's not the first time we're hearing about this. In December, thanks to a leaked email allegedly sent between Sony execs, it was mentioned that "the Z4 phones" were planned for May / November 2015.
Starting with the original Xperia Z from early 2013, Sony introduced a new flagship once in about 6 months. Thus, we had the Xperia Z1 in late 2013, the Xperia Z2 in early 2014, and the Xperia Z3 in late 2014. A "summer" release for the Xperia Z4 certainly doesn't fit the pattern, but perhaps Sony just isn't ready to introduce its new flagship.
Feature-wise, the Xperia Z4 could offer displays with both Quad HD and 1080p resolutions, an octa-core Snapdragon 810 processor, and a 21 MP camera with a new Exmor RS IMX230 sensor.
What do you think, is releasing the Xperia Z4 in mid-2015, instead of in the coming months, a good idea?
Alleged Xperia Z4 renders (their authenticity can't be confirmed yet)
source: Xperia Blog