Okay, time for the obligatory Dr. Dave reference...
If you guys take a look at
this page, you'll find several descriptions of alphabetical player ratings, with quite a bit of variation in their descriptions of each rank, so there's bound to be some difference in what everyone here considers a "B" player.
The point I was getting at before was that there seem to be certain tipping points where you can tell that someone is progressing in their ability as a pool player, so I thought it was interesting that there are very few players with a 4-pack as their high run.
I also think that someone's all-time high run doesn't tell you too much about their ability as a player since it's a combination of skill, luck, and opportunity that produces large packages. Sure, you can probably infer that someone that has strung together 5 racks can play a bit, but without knowing how it happened and what the equipment was like, it could just be a fluke.
Now for some back-of-the-napkin math...
Let's say the average pro breaks and runs 1 out of 4 racks of 9-ball, which seems reasonable based on past tournament statistics. Their chances of stringing together 5 racks beginning with any given rack are (1/4)^5=0.9765625, or about 1 in 1000. Looking at that a different way, it is
reasonable to expect to see a 5-pack (or better) from the average pro for every 1000 racks they play.
What if you're not that good and
only run racks half as often as the average pro, so 1 out of 8. Your odds of a 5-pack are only (1/8)^5, or 1 in 32,768. In fact, if you run out 1 of every 8 racks, you should only expect a 2-pack once in every 64 racks you play, at least according to the numbers.
Anyone that has a good idea of what their B&R percentage is could do a similar calculation. In reality, I would guess that the actual occurrence of packages is more frequent that these simple statistics suggest because of some of the factors previously mentioned: figuring out the break/rack, the mental/emotional state of the player, etc.