Yeah, I know starting with a developer preview is probably not a great tactic, but I'm generally a fairly competent programmer, and I'm eager to get started.
Anyway, I found out that the build tools aren't really in place yet, so you just need to copy a few files over to the proper directories and you're good to go.
I created a Github project to act as a starting point for anyone who just wants to play with Ext JS 4.0. I'll try to keep this updated as newer versions of the developer preview and or beta are released. I would love your thoughts/comments, so just post a message here or add an issue on Github and I'll look at it right away!