The 4 sites (other than meta) are the 4 top sites in terms of number of successful migrations. Martijn's answer gives a better overview of how they are actually determined.
Programmers, Webmasters, and Server Fault used to be on this list, and unfortunately SO community members routinely tried to migrate crap because "this is subjective, might be better on Programmers", "this is about IT, it must belong on Server Fault", "This is about running a website, probably better on Webmasters". In the end these ended up with a lot of rejected migrations, which contributed to removing them from the list of migration paths.
Update Migration Path List for Non-Moderators shows the stats and ranking from mid-2012 when Programmers and Webmasters were removed. Programmers came in 6th and Webmasters can in 10th as you can see. And in November, Sharepoint was added in favor of ServerFault.
If you feel there is a really high quality question that was just asked on the wrong site, Mods can still migrate anywhere (as long as the other site agrees). In that case, you can suggest a migration by flagging with a custom message as to where it should be migrated and why.