We had DAI three and a half years after DA2, and that was working with a whole new engine. Aside from this "one game per year" rule, why would it take five years for a new DA title? Do we even have any indication what the new IP is, let alone that it'll launch in 2018?
For what it's worth (which is absolutely nothing, since I'm just guessing) the reason I'd bet on a 2019 date for DA4 is that only has a skeleton crew right now, whereas the Sekrit New IP has a whole team and has been in full production for quite a while. That makes it seem as though the Sekrit New IP, whatever the heck it is, will launch before DA4.
This isn't the same situation as DAO to DA2, or DA2 to DAI, where they kept a larger team working on the franchise throughout. I'd expect at least two years of full production for DA4. They might make November 2018 if they ramp up in the next few months, but that seems like the absolute earliest plausible date to me. Considering that DLC for the Sekrit New IP might still be coming out by then, my entirely imaginary money is on March 2019 instead.