0.4 is a considerably major expansion to 0.3, where Jon002 figured out how to load worlds. It is largely based off of partial (but major) expansions to the server by humanoid24. However, only 5 world checksums were known at the time (Nimbus Isle, Nimbus Station, Crux Prime, Gnarled Forest, and Ninjago Monastery).
It was this release that triggered the isolation of official releases by humanoid24 and lcdr, and the attacks by Raffa505 (DMCA x2) on Timtech Software. However, a counter-notice was filed due to the fact that the files were licensed under the General Public License (version 3). Google has not responded further, and the pages have currently been removed from Google since late 2013.