So after looking at my list that I made yesterday, I found out some observations that may be interesting, here's what I found.
Shantae is particularly popular over here in the West, many fan polls have shown that her popularity unmistakably resides here, and what separates her from other characters are her various animal transformations, which would be a first for the Smash Bros series. She also now has a publisher in Japan, meaning her licence would be easier to acquire.
K. Rool is popular all over the world, however his popularity usually resides in Europe, the country he was created in. He usually destroys fan polls over there, and what separates him from other characters is his multiple personas, so it's like playing four characters in one, which would be a first for the Smash Bros series.
(Bandana) Waddle Dee is particularly popular in Japan, usually ranking very high in fan polls over there, and what separates him from other characters is his use of his spear, which would be a first for the Smash Bros series.
Cloud is a very strange one, as his situation is quite different from the others. From what I could tell he was popular internationally, but thing was that literally almost no one expected him and considered him a pipe dream. So he must've been the "Surprise" character, in other words, the one everyone (exaggeration) wanted, but no one (exaggeration) expected. The thing that separates him from other characters is the Limit Break mechanic, which boosts his abilities, which is technically a first for the Smash Bros series since Shulk's Arts boost him in one area while Cloud's Limit Breaker boosts him all-around (at least from what I've examined)
All four of these characters are popular around the world, in fact Shantae is the most popular indie character in Japan and Europe according to fan polls, not to mention all have something that no other Smash character has, so their popularity and abilities would most likely be taken into account. Because of this, I think that they were decided because of these factors...
1) Popularity
2) Uniqueness
3) Accessibility
And those are my thoughts. We'll see what'll happen soon in a month.