BBallTim...
The 4th player could be basically anyone playing center that has a high fg% (Shaq, Perk, JO). The whole point, is that Rondo is surrounded by 4 players that either demand a double team, or consistent attention. On a lesser team, defenders would be able to collapse on Rondo if he penetrated. No other team surrounds their PG with as much talent at every position as us.
Since Rondo's only played significant minutes with the big 3, it's impossible to say how he would be at driving and finishing on a different team. The whole premise of the argument I think D'Antoni would make, is that on another team, he wouldn't have the spacing he does with the C's, and that would limit the effectiveness of his ball-handling, passing, vision, etc. He'd certainly get to the line a lot more, but for him (and his team), that's usually a negative.
The whole point, is that playing on the C's capitalizes on the strengths Rondo has, but on a lesser team, it would be the skills that he doesn't have (shooting), that would be emphasized.