Again, if it makes sense to you that you should only see 4 of a kind once in every 7,000 hands, then you probably shouldn't be trusting your judgement as to what seems or feels like strange results. That's not a criticism of you personally - most people don't have the greatest intuitive grasp on odds. But if that's the case, probably best to make sure one completely understands the odds before coming to conclusions.
So I assume you've based your assertion, rather than on the tables I gave you a link to, or on the wizardofodds tables linked to by the first person who responded in that other forum, on his grossly incorrect interpretation of said tables. It would appear he's taken the provided probabilities, and turned them into percentages, so his numbers are off by a factor of
100. The table "Texas Hold 'Em -- High Hand Probabilities -- 6 to 10 Players", shows that with 9 players, the probability of one player having quads is .013183 - or in 75.9 hands. Not 1 in 7,000 - 1 in 76. Now, if you instead meant that you personally have received quads 20 or 30 times, of course that happens less often - 1 in 595 I believe would be the correct number for that.
Of course the assumption is that no one folds, which won't be the case in real life, so the numbers in reality will be a different - but not 10-100x different.
If you read further on the same page, you'll see at least one other person provided the correct answer.
If what I'm saying to you about the table doesn't make sense, think of it this way. Every possible hand has been provided. If you add up all the probabilities in any one column, they should add up to 1. If they were percentages, as that poster suggested, that would be 1% - so what hands come up the other 99% of the time?