Kaka' doesn't have as great or intense a body of work as the others to be compared to them. He and Ronaldinho, one might say had very short peaks, but if that is considered the case for Ronaldinho, we can say he indisputably shone brighter and his measure at the time he was the best player on the planet, was literally himself, i.e. outdoing his own outrageousness with something even more jaw-dropping.
His ability to run with the ball and actually use it correctly with both technique and intelligence at his absolute top speed is matched by few players, and the knowledge he had to be stopped before he got into his full stride made him a tactical nightmare for the opposition because he picked his moments to spring into life in a manner very few extremely quick dribblers do or did - i.e. with such a clear and obvious weapon, most players will use it at any and every opportunity, but Kaka' would optimise himself and his runs to goal to make them the most unexpected they could be. Just a very clever player.
Unfortunately, like so many other extremely fast runners, his knees simply could not cope, and when they went, he lost his edge because, although not reliant on pace, the mere threat of him bursting out the traps kept opposing players honest, uncertain and positionally error-prone (standing too far off him, or getting too tight and giving him direct lines behind said player to exploit), which enabled him to play without restriction or consideration, in the sense that these things were a given - the opposition had to fear his pace and acceleration, it wasn't optional. So with his legs gone, all of this evaporated and Kaka' couldn't adjust in the way Ronaldo did, as an example.
It's a tragic loss as Kaka's story had only reached the exposition and we didn't get to see it told as the writer had intended - I feel his legacy would have been a notch or two higher had his injuries not occurred because his intelligence married and entwined with his game to a world class level, which wouldn't have been 'found out' rather, having an organic conclusion when he hit his degrading pace years.
As others have stated, Kaka's technical level was not on par with the others mentioned, which is no slight, given we're talking about magicians and conjurers here who could execute pretty much whatever came to mind at the drop of a hat - Kaka' couldn't do that; his execution was linear and predictable by comparison and he had to fashion his windows of opportunity in less improvisational ways. You won't see the same kind of array of plays from him as any of the others because it wasn't in his locker relative to them, but what you did know is, like Robben, what he was gunning for was going to be very difficult to stop even if the path he was taking to work the opportunity was clear and apparent.
I'd also say Kaka' was never 'the' man or the sole hope of his team in the heroic fashion that can be ascribed to the others. Sure, he won a WPOTY, and he could produce sublime moments, but he couldn't concertedly distract swathes of opposing players over 90 minutes over picking and choosing his defining moments. Ronaldo, Ronaldinho, Rivaldo and Romario all had the pressure and expectation of being full-time distractions against any opposition under any conditions and circumstances, in a manner and spotlight lesser players wilt under. It could be said that this pressure ruined an even brighter legacy for Ronaldinho as 'his' World Cup '06 never materialised at a time when it was supposed to be his crowning glory. Kaka' never had a 'his' anything, which is a bigger degree of separation for him than anything else I've written apart from possibly his knees. If you're not that guy in such company, then you're playing second fiddle to it, which was where Kaka' ultimately got to during his pre-injury days.
All of this is relative, however. Comparing most anyone to such names will have a similar outcome, so it's not a slight on Kaka', more a reflection of what he is being matched up with. He was a seriously impressive player who did the best his ability permitted him to do until his career was curtailed. I don't know if he could have contested for more WPOTY's because the eclipse was just around the corner, but I doubt his place as a constant top 5-10 player in the world would have ever been in jeopardy whilst he was still anything like the athlete he was.
. Because of his flop at Real Madrid I think that all the above players are considered as better.
