Why is it called 4d2.org? Does it mean/stand for something?

It's mostly just a short domain name that we thought was cute and relatively easy to spell, with a sideways reference to the number 42.

When Sarah registered 4d2.org back in 2001 for a personal hosting project, the newer gTLDs didn't exist, 2-character domain names were exhausted, and 3-character domain names nearly so. Some poorly-configured mail servers would still refuse messages from domain names beginning with a digit, despite RFC 1123 having been around since 1989. So, many of the remaining 3-character names began with digits.

Since 4d2.org was to be a nonprofit project, the decision was made to follow the traditional .com/.net/.org structure and choose a .org domain name. Two years later, .org would be spun off from Verisign, the successor of Network Solutions (the first DNS registry operator). Verisign agreed to cede .org to a new nonprofit, the Public Interest Registry, in exchange for permission to continue rent-seeking with operating .com.

4d2 was chosen because "4D2" in base 16 happens to equal "1234" in base 10, and in most American accents, it sounds a bit like the answer to Life, the Universe and Everything - /ˌfɔɹɾiˈtu/. Sorry there's not a more interesting story there.