Kirby is a nice person and the main protagonist of his own self titled Nintendo series, it's obvious that Kirby will appear in SSB4 because the creator of Kirby made this game (see below) this game.Plus Due to him being in all 3 previous installments of the series.The creator of the Super Smash Bros. series is also the creator of Kirby, this increases the chances of return.
He appeared also in the whiteboard drawing , that Masahiro made on 2 July 2012 .
The only change to Kirby that seems likely are having the new "Hats for new characters " Kirby's Model base was use to create Jigglpuff. This is probably why some of his moves are similar to his such as the third taunt or tilt up. And a intersting fact is the japanesse version of jigglypuff is Purin. In the Japanese version of brawl when Purin uses his B move Roll out she will say Pu...rin when spinning. And Kirby will saw Purin too only in any language brawl when he wears jigglypuffs hat. He is also seen in all Smash Bros games.
Kirby's final smash is Ultra Sword