They've been a constant in the domain sales reports at [
dnjournal.com ] .
I registered a nifty little "digital domain" (0 1 0 1) that I flipped about a year ago. I registered it because I thought it was a) cute and b) carried an implied "this is about digital" message.
4 digits -> sold for 5 digits. I wish I picked up more, but as I recall, there weren't many numericals (4 digits) available at that time.
Now, about those 4 letter domains . . . It seems like that space is also heating up . . So you may want to nose around a bit in the auctions "on the cheap". You could pick up decent ones for ~$50 or less, five or six years ago. By decent I mean ones that weren't all qzxc or xkzf or fzqx.