Xiang,
No worries. How many devices and what sort of custom work have you loaded into your Zenoss? Your performance data isn't stored in the ZODB, so that will be uneffected. You'll want to backup $ZENHOME/perf as this is where all the RRD data lives. If your setup is fairly stock I'd almost suggest you setup a second server running the same platform, reinstall zenoss and your zenpacks then rsync over $ZENHOME/perf. Once you do that you can systematically add your devices back in to Zenoss. As long as the devices are added using the same ID as before they'll automatically point to their correct performance data directory under $ZENHOME/perf.
If you need to dump your ZODB to rebuild I generally reccommend writting a short zendmd script that outputs every devices id, title, deviceClass, groups, systems and manageIp into a CSV or XML doc which you can then read with a simple python script to add devices back in. You can also use batchloader, though I'm not 100% positive it works in v4, I can't claim to have tested it recently.
--Shane Scott (Hackman238)