As a Kirby player, here are my general thoughts about how he was in each game before I go into detail about Smash 4:
Smash 64: The second best character next to Pikachu.
Melee: Worst character, and yes, I'm butthurt over it. :/
Brawl: Since Brawl was the first game in the series that I played, Kirby was the first character I grew attached to. He's pretty good, which is saying a lot for Melee.
Smash 4: Kirby is more or less the same as he was in Brawl, but with some tweaks. Kirby's biggest strength is his incredible combo and recovery game. Since Kirby has 5 jumps, recovering with Kirby is going to be easy. Kirby's combo game is just amazing. His Down Air is by far his best move, as it can lead into several possibilities from jabs, Smash attacks and grabs. Kirby's grab game is also really good, with an incredibly fast pummel and decent throws, with Up Throw being an okay kill throw at later percents (around 160-180, I believe). His Up Tilt is another great move of his, since it's one of those tilts that locks an opponent, like Fox's U-Tilt. Kirby's air game is also great, with his B-Air doing a lot of damage and his U-Air being a good move to keep opponents in the air. I already mentioned the awesomeness of his D-Air, but it can also cause a mini-spike in the air. He has pretty good specials. His N-Special can allow for some interesting match ups, as it can change the tide of the match. The U-Special can be a decent spacing option with the little wave it shoots out and can spike as well. D-Special, Stone, is a risky move, as missing with it can allow for a bad punish since you can be grabbed in that state. Only use it to catch your opponents off guard in the air. Hammer Flip is only good for a punish or after a shield break, but if you're confident that you'll hit with it, try. Kirby's Smash attacks are extremely good, especially his Side Smash, as it has a lot of knockback. As far as Kirby's weaknesses are concerned, since he's floaty, he slowly falls, which many opponents can use to their advantage to keep Kirby in the air and punish him. This is obvious, but since Kirby's one of the lightest characters in the game, he's gonna be destroyed pretty quickly unless you know what you're doing. Overall, while light and floaty, Kirby excels at his air and combo game, making him a threat in the right hands.