http://www.openlaszlo.org/lps/laszlo-explorer/index.jsp
This page has links to useful documentation. If you are unfamiliar with Open Laszlo, going through "Laszlo in 10 Minutes" is a good idea.
Next, the Developer's Guide, http://www.openlaszlo.org/documentation has more in-depth information on coding in Open Laszlo. I'd recommend getting the Nightly Build version of the Software Developer's Guide, as it will have the most up-to-date information for the OL4 release.
The forums are a good place to search for information on specific issues, or post questions: http://forum.openlaszlo.org
There is also a Wiki: http://wiki.openlaszlo.org/Main_Page
There are some excellent example applications, with source available, at http://www.openlaszlo.org/demos; I'd particularly recommend looking at LZPIX.