Time

Lately I've noticed that I have a hard time judging long time spans. Here is an example:

Warhammer 40,000 is going to be coming out with a new edition this summer. This will be it's fifth edition. When I heard this I sent an email to my friends commenting on how weird this was since they just came out with fourth edition a little bit ago--I was thinking like two years.

My friends responded by saying that the fourth edition has been out for quite some time. So, I was kind of confused. I went to my fourth edition rulebook, which I bought when it first came out, and checked out the copyright date... 2004. It came out four years ago. How did I miss two years?