When big pile of batch in -Current came out few days ago, a new package for bash 4.0 appeared for the first time in Slackware testing directory. So far, i have been using bash 3.2.048 for some time and i didn't encountered any major problem. I even used this version to teach students who took my class (Unix System Programming).
Since i thought bash 4.0.0 is quite stable, i tried to upgrade to this version along with other updates as well (KDE 4.2.1, Glibc 2.9, GCC-4.3.3, etc). I tried several simple commands and it stopped and i couldn't do anything about it. I had to close the console and started over. I repeated this several time and produces the same output. Also when i use tab (for auto complete feature), it crashed.
My conclusion for now after using bash for about five minutes is that it's not yet stable and ready for prime time. I encountered the same problem noted in Ubuntuforums.