Well, well, well. A small bit of...odd...information has just been unearthed regarding the great King himself and Super Smash Brothers Brawl that might just be the final boost of support that K. Rool needs.
If you go and check out the last couple of pages in the SSB4 Rumors thread, you'll find that a leak was posted where a kid used the Way Back Machine (a program that archives old and deleted webpages, and allows you to visit old snapshots of webpages) to check out the character URLs for the Smash 4 website. The video can be found here:
https://www.youtube.com/watch?v=oTqTvJaZukE
Obviously, this leak is fake (Geno, Neku, Ocelot, Trails???), but this lead people to check out the URL history for characters on the Japanese website. Not much was discovered other than a ton of Brawl dojo related content...but there was one thing that REALLY stood out...to me at least. There is a character page listed for King K. Rool. Here is the link to the page where it is listed (his name can be found in-between King Dedede and Kirby):
http://web.archive.org/web/*/http://www.smashbros.com/jp/characters/*
EDIT: The listing is on page 22.
Of course, the page isn't view-able. It most likely doesn't even exist anymore, but the fact that it is listed in this archive means that it existed at one point in time. The question is: why?
Could King K. Rool have been planned for Brawl, but scrapped like other characters? If so, it would be strange that no character data for him can be found in the Brawl data, while others such as Mewtwo and Roy can. Furthermore, why does he have a character page while other characters who were further along in development do not?
Personally, I believe this means that K. Rool was indeed a planned character for Brawl, but just didn't make it cut. Now, we all know that Sakurai likes to revisit old ideas (as seen by Villager being planned for Brawl, and now being in Smash 4). It wouldn't be a surprise to see a character who was scrapped from Brawl appear in the new game. With all the other bits of information, such as K. Rool's immense popularity and demand, the massive resurgence of the Donkey Kong series, and the Kritters appearing in the Smash Run mode, I think it's safe to say that K. Rool will most likely be playable in Smash 4.