Oh damn I didn't even think about specifics when I posted it.
Well, if I were limited to four, I'd want to pick a diverse set of characters that are:
A) Iconic in their own right (like, characters you'd see a lot of folks dressed as at a con or on Halloween)
B) Come from popular properties (high ticket sales, lots of merchandise, Parks presence, etc)
C) Have abilities that can be easily transferred to a Moveset
D) Come from different corners of the company
E) Obviously, I need to like the character, since it'd be me choosing it.
So a while back I made a list of characters for a Disney Smash Bros and I went back to look at it. I took the ones I could easily make movesets and play styles for and put them on the master list. By then, I had quite a few, so I further cut it down based on the above criteria. I still had a few, so it was basically Sophie's Choice for me at that point.
In the end, I went with:
Mickey Mouse - Represents Walt-era Disney animation and is the company mascot. Attacks with multi-purpose magical abilities due to him using his Sorcerer Mickey appearance, which is the most apt for combat. Can summon brooms to help as well. Final Smash would be Fantasmic, in which he performs the same magical attack he uses at the end of the Disney Parks show of the same name.
Elsa - Represents modern Disney animation. Attacks with a variety of ice attacks and focuses on whittling the opponent down. Can slow foes with a flurry or change the terrain to ice to lower their traction. Her Final smash would, of course, be to summon Marshmallow to crush opponents.
Captain America - Represents Marvel. His shield is the most versatile projectile in the game, with its trajectory being able to be altered mid flight, as well as maintaining momentum and being able to ricochet off walls and foes. Taking a page from Disney Infinity, his Final Smash has him hold up his shield as a S.H.I.E.L.D. helicarrier launches a huge *** laser at it, and Cap deflects it to heavily damage opponents.
(Captain) Jack Sparrow - Represents live-action Disney as well as the Disney Parks. A swashbuckling swordsman who also has a flintlock and bombs hidden in chests, a la Kingdom Hearts. Being a lucky, oft-inebriated wildcard, he excels at mix ups, with his move start ups looking suspiciously similar before switching to a less guessable attack, keeping foes guessing as to what he'll do next. His Final Smash would have the stage become sea level as the Black Pearl appears in the background, launching a rain of cannonballs on everyone on the stage.
For the record, if I could choose a fifth, it'd of course be Darth Vader, but he was just a victim of the numbers game. There'd also be Assist Trophies for Buzz Lightyear (getting Pixar in the mix) and Darth Vader, since he got snubbed of playability.
TL;DR, picking four was really hard.