It's hard to say exactly how many times we've blown a 4th quarter lead. However, clutch efficiency ratings are readily available. For this past season (courtesy of a friend that follows the Mavericks), here are the relative clutch efficiencies of the various NBA teams that made the playoffs against each other:

The blue bars represent offensive efficiency in late game (less than 5 minutes left in 4th quarter or overtime, with neither team ahead by more than 5 points) situations, and the orange is defensive efficiency in the same categories.
As you can see, defensively we are average, pretty much exactly in the middle (Chicago is the best, Dallas the worst). However, this actually indicates poor performance, as that indicates we've dropped from the best defense in the NBA to an average one in clutch situations.
Offensively, we are just slightly higher than average, on par with the Spurs and the Lakers. The best are the Magic, the Mavericks, the Trail Blazers, and the Suns. The worst are New Orleans and Chicago.
Unfortunately this data is only for this post-season and not previous ones. However, it seems to me to be fairly accurate. In clutch situations we regress to the level of a low-seeded or average playoff contender, as opposed to the elite team we should be, especially defensively.
The other way to look at it is to go to 82games.com's list of the best individual clutch performers this season,
here. Our best clutch performer is, no surprise, Paul Pierce ... but he ranks at 39th in the league. The next Celtic to appear isn't until Ray Allen at the 56th mark. In addition, Rondo was the 9th WORST in the league (or at least among players that 82games.com analyzed).