The first builds of 4.0.0 did have some bugs that caused crashes. These defects are being traced, and many of them have been fixed in the newest build. Make sure that you're using the newest build (currently Build 6), or the newest 4.0.1 beta (currently beta 6). If you are using 4.0.0 build 3 or older, upgrading to newest build will most likely help you.