With all due respect, I really hope not.
I'd be shocked if we saw a fifth park anytime soon---though I bet they have at least two and maybe three plots already in mind for one if it comes to that in the master development plan.
As Tim points out, standalone parks take an enormous amount of infrastructure and fixed costs before you ever get a return on investment. It would be much less expensive to add capacity to existing parks---and all of them have room.
In the MK, even after the 20K plot is consumed by Fantasy Forest, there are good-sized expansion pads in Tomorrowland (Carousel of Progress through the old Galaxy Palace theater) and in the southwest corner of Adventureland.
EP has room for several more country pavilions, the old Wonders of Life is emtpy most of the year, and the convention space over behind UK is underutilized as well.
AK has all of camp Minnie-Mickey available for expansion; there's a big plot there and very little of it is used.
DS is the one that is most severely landlocked, but a very sizable chunk of that park used to be a working studio that isn't a working studio anymore. Lots of room.
The only reason to add a new major gate (as opposed to adding capacity to the existing ones) is to try to extend the length-of-stay. And, if they really wanted to go after more of the UK market, they might, because the UK guests often come for a couple weeks. But the US market doesn't typically vacation for much longer than a week or so at a time, and already there are enough things to do at WDW to keep week-long guests busy.
If a new gate is built, I think it is much likely that we'll see other minor gates; maybe another water park, as both often reach capacity in the summer, and they are harder to expand as both sort of have their boundaries set by the lazy rivers. Or, maybe the Night Kingdom idea---a competitor to Discovery Cove in terms of price point and service---might resurface. I think either is more likely than a major gate.
and possibly a ski lodge could work, too. 
