So, you know, Movable Type was released 4 years (and 1 day) ago, & to me, that's pretty astonishing.
This has been in the back of my mind all weekend , but I couldn't really think of what to write.
So what got me thinking tonight was reading Nick's post about Movable Type, and realizing (once again) what is, in retrospect, pretty obvious: that this is about so much more, & involves so many more people, than when Mena & I first released MT in 2001.
So I think that, for me, that is what marks the 4th anniversary of Movable Type's release: the people we have at Six Apart, the people we've inspired in the community, and all of the people using our tools (Movable Type, TypePad, and LiveJournal) in general.
Or put another way: what marks this 4th anniversary most, for me, is the fact that it's not only me, Mena, or Anil writing about it. It's not just us remembering how much we've changed in the past 4 years, and how weblogging in general—and in a lot of cases, Movable Type, TypePad, or LJ, specifically—has played a part in that. And there are so many more than just us who are coming to work every day & working to build tools for the future of blogging. 
For me, one mark of the success of any software or service is its relative ubiquity. At first, the fact that people used MT without knowing about its history used to feel a bit weird, because it grew out of such a personal need, and Mena & I always felt so connected with it. But the fact that there are millions of people using Movable Type who don't even know or care that this is the 4th anniversary of its release is, in a way, the best indicator we could have about how much it's grown in 4 years, and continues to grow.
What Movable Type's release 4 years ago symbolizes most to me, in other words, is the beginning of everything that's led to today, something that I always marvel at, and kind of have to pinch myself about: that Six Apart is a company of about 100 people, around the world, who are doing great things, & who I'm incredibly proud and happy to work with.
So, happy birthday, Movable Type, and Six Apart by extension!
 Yes, an obvious misquote, but what was I to do? I couldn't think of any good quotes or lyrics or song titles about 4 years, so I had to remix (how Web 2.0!) another.
 Of course, some of this applies to last year, too—but I'm comparing all the way back, to 2002, 2003, etc. And things have changed a lot.