Intel has made a very big deal about the fact that the last generation Haswell chips and the new Broadwell chips are "socket compatible." This means, in theory, it should take relatively little work for Microsoft to just take the latest batch of Broadwell chips from Intel and launch a more powerful and power efficient Surface Pro 4.
Other than the new chip, these devices could be physically identical to the Surface Pro 3.
If Microsoft goes down this route, then I would expect that the software giant to launch the "Surface Pro 4" (if it ends up actually calling it that) sometime in the next month or two. This would presumably last until Microsoft launches the real "next generation" tablet powered by its new Windows 10 operating system and Intel's new Skylake processors.
The Surface Pro 5 could be the big improvement
In moving from Haswell to Broadwell, Intel brings significant power efficiency improvements. In theory, this should allow for a sleeker device while offering the same or even higher performance than the prior generation Surface Pro 3.
However, given that a thinner/new chassis would necessitate the design of a new motherboard, it probably makes sense from an R&D efficiency perspective to wait on the full redesign until Skylake, where a new motherboard would be required anyway.
At any rate, I'm expecting Broadwell in Surface soon
Since Microsoft has repeatedly claimed that the Surface Pro family of tablets is quite suitable for replacing a laptop, and since most thin and light laptops will likely transition to Broadwell quite shortly, Microsoft is very likely to update its Surface Pro lineup with Broadwell.
In my view, the biggest unanswered question is simply what Microsoft is ultimately going to do about branding. There is obviously precedent within the tablet market, and within the Surface family of products, for relatively short product cycles, so a Broadwell Surface Pro 4 in February and then a Skylake Surface Pro 5 in July wouldn't be unheard of.
At the same time, customers who may not be privy to Intel's public statements that it plans to ramp Skylake this year could feel annoyed at having bought the "new" Surface Pro 4 only to have it obsoleted relatively quickly.
I look forward to seeing what Microsoft ultimately ends up doing, but my guess is that any Broadwell-based Surface Pro product will be quite short-lived.
