The Sony official didn't say exactly when the Z4 handset would be released, but he mentioned that the company hadn't moved to a 1-year release cycle - thus, we should expect to see the Xperia Z4 hitting shelves before September (as you may know, the Xperia Z3 was introduced in September last year).
Speaking of the Z3, Tim Harrison says that the entire Z3 series (including the 5.2-inch flagship model, the Z3 Compact, and the Z3 Tablet Compact) is "doing really well in the market, and there is still quite a considerable amount of interest in those particular products."
Another Sony official, Pierre Perron, Vice President for Western Europe, told The Inquirer that the company won't launch a new flagship smartphone "just for the sake of it."
The bottom line is that, for now, Sony can't tell us when the Xperia Z4 will be released, but it's likely that this will happen in the summer - just as previously rumored. The new flagship smartphone should be powered by a Snapdragon 810 processor, and might have to versions: one with a 1080p display, and another one with a Quad HD (1440 x 2560 pixels) screen.
sources: Trusted Reviews, The Inquirer via Xperia Blog