Are you saying they could build a 5th park within 10 years of announcing it? I think that's a stretch. It took them like 5 years to build New Fantasyland after announcing it, and that's just this tiny piece of land. Star Wars Land was announced in 2015 and we all expect it to be finished in like 2020, which is 5 years. It's going to take one year to build a Frozen ride. One entire year for one ride? An entire new theme park would probably take Disney 15 years to build.
So if you assume that they wouldn't announce a 5th park until Avatarland, Toy Story Land, and Star Wars Land are completed (at the very earliest), you're looking at 2035 before the new park would open. 2020 Star Wars Land is finished, plus 15 years to build the 5th park = 2035 opening date.
So, sure, there COULD be a 5th park at some point. But none of us will be alive to see it.