Those are the release dates for volumes 1-3 (according to Amazon). It's been 5.5 months since the release of volume 3, hardly "a long time" considering the 363 days between volumes 1 and 2, and the 5 months between volumes 2 and 3.
Many shows have 6-12 months between releases, so 5.5 months really isn't that long. Volume 4 will probably be released early in '06, I would imagine.