About a month away:
From 4,000 U.S. deaths in Iraq
. Because it's a round number, a "hard number" (i.e. not an estimate), and a substantial number, it will be invoked frequently this year.
Respectfully disagree if your implication is it will have any impact on the discourse other than a half week of discussion on CNN, and a 75 second mention on network nightly news.
1000, 2000, 3000 all came with the implied promise that NOW people were going to stand up and take notice and do something. Such numerical milestones have had almost no effect on the overall debate. People who are for our continued presence in Iraq - William Kristol and the 19 percenters - will be completely unphased. The majority who are against McCain's 100 year war will have a few days of sadness and fury and feeling powerless to convince the slim Dem majority to do a damn thing.