Being on TC isn't just important for the numbers. It's who those people are. I suspect practically every startup investor reads TC, for example. That's not a factor for News.YC, but for the average startup those are good people to have know about you.
Also, the traffic boost is probably ordinarily more than 4k uniques. We probably have a fairly big overlap with TC readers.
Traffic boost is not normally much more than 4k in my experience. I've had two startups on TC (multiple times each) and worked at a third startup that also got coverage. Ranged between 3.5k and 7k uniques (so you're right-- it's on the low side).
Regarding investors, I think you might be right there-- TC coverage probably helped our YC app, for example (you'd know more than I!). But, in conversations with investors, I've NEVER had one say "Oh yeah-- I saw you guys on TechCrunch." It's a nice bullet point in an email/execsum that lends credibility.
Arrington literally has talked about founders calling him crying and begging for coverage and busting into his house to plead with him. That just seems wrong to me.
To provide an opposing anecdote, our startup literally got direct contacts/calls from VCs specifically due to our coverage on TechCrunch.
We had been front page on Digg and that got us a crazy number of uniques, but the initial VC interest did not start until we were on TechCrunch and Lifehacker a week or two later.
Also, the traffic boost is probably ordinarily more than 4k uniques. We probably have a fairly big overlap with TC readers.