Just because he has a high rating in the 4th quarter in games doesn't mean he is great under pressure...
Take a look at his fourth quarter splits when the game is within 7 points, then, and get back to me. Or his half-time splits in the same situations. Tony Romo is good under pressure.
It'd be interesting to see just how many deciding 4th quarter series he's been put in relative to other QBs. To see how often he's put in a score-or-lose situation, scores, only to have his defense give up the next series and put Tony back in the same situation only with less time to be successful. This might account for him having the high rating and also the (relatively) low success rate at pulling out the games. And why we see so many bizarre stats where he's the only guy in the league to have 300 yards passing, 3TDs and no picks and lose, or whatever those numbers are that percyhoward trots out from time to time to make us all feel bad.