I've got more than a handful of 4 number .net's and .org's , and I've posted some in the expired forum also.
My personal take is this:
As I mentioned earlier, I've never had a 4 number .net that hasn't paid for itself in parking ( 7125.net registered about a month ago, already $10 in parking, not a bad ROI ). This is mostly because all 4 number .net's were previously held by someone (Evidence: They are all gone now, and they've all been gone before) so you have built in backlinks typically. A lot of the 4 number .net's were held by asian owners, so the traffic might not be the best ( 5725.net gets a ton of uniques, but mostly poor traffic )who can't read the parking page

They are relatively rare, with less than 9,000 available to begin with.
It is true that I cherry-pick for 4 number combos that are good years or have a lot of 5's and 0's in them (Easy to remember, money combos, etc.), but so far I've been pleased. I don't remember ever selling any 4 number .net's (yet

), but compared to some of my other purchases, based on ROI they're doing just dandy
-Allan