Astute reader '420guy' has noted in a comment thread here that Jackson left some juicy tidbits in the comments of his LiveJournal. Specifically a status report of sorts on the much anticipated (but always elusive) season 5 of Venture Bros. When asked by a loyal reader for a morsel from the season:
Sorry, man... nothing to show until the summer. We're just about to ship the first episode to Korea to be animated.
Which, to me anyway, implies you guys will get to see some stuff during convention season. The big three conventions, as always, are San Diego Comic-Con, Dragon*Con and New York Comic Con. Jackson and/or Doc are likely to make appearances at all three of them, possibly with clips and trailers.
Perhaps even more telling is the fact that the first episode was just shipped off to Korea (his comment was written on April 8th), meaning there are still a good seven or eight months before that episode is completed, which already puts us into early next year. Coupled with Jackson's other comment (in response to asking for the title of the season 5 premiere):
Sorry, but we haven't actually written the premiere episode yet. And since Doc wrote the "first" episode in the production order, I can't tell you the title. Because he totally hates when the titles get leaked.
Obviously the season can't begin until the intended premiere is completed, and even if Jackson and Doc suddenly wrote a script for it today there would still be around a month of pre-production, plus another 7 - 8 to get the animation finalized. So, my own (hopefully) educated guess is that season 5 will start sometime early next year, somewhere around March. The best I can do is speculate, though, so take that with some grains of salt.