"There's no such thing as a [Surface] Pro 5," said Panos Panay, corporate vice president for Surface at Microsoft, told CNET last week. Microsoft won't update the Surface Pro 4 until it can make a change that's "meaningful," with Panay looking for "an experiential change that makes a huge difference in product line."
This comes in spite of Surface sales tanking hard in the first quarter of this calendar year, with the hardware's staleness a growing problem. Microsoft hasn't updated Surface Pro 4 for 18 months now; the systems still use Intel's Skylake processors, rather than the latest Kaby Lake chips. A switch to Kaby Lake wouldn't be huge—though it would help battery life across the board, would give the bottom-priced Pro 4 a decent performance boost, and support hardware-accelerated 4K video—but it would go a long way toward showing that Microsoft still regards the system as a going concern.
Panay's words (probably) don't mean that there is literally no Pro 5; as with most public statements of this kind, what he means was, "there is no Pro 5 that we're willing to talk about or acknowledge at this time." Right now, the company wants the focus to be exclusively on its new Surface Laptop, and talking about Surface Pro 5 would both distract from this and risk further reducing Pro 4 sales.