I have been investigating just now what the status is for the minecraft server development and 1.4. Generally there are good news. Here is what I found:

• There is a craftbukkit build for Minecraft 1.4. This means that plugin developers have the chance to adjust their plugins accordingly. This means that hopefully over the weekend, all essential plugins will be up and running with MC 1.4
• I tested that craftbukkit release with all plugins. Some work already, others do not.
• Worldedit and Worldguard (the most important plugins) are also broken. The developer made an announcement that a version working with 1.4 will be available on Friday. This will also open the (real) darkworld, the railway and a bunch of other things I have been waiting for since more than a month now.
• I will bring the server to 1.4 as soon as user levels and protection is working. Stuff like minecarts, shops etc might come later.

So if you upgraded, I would assume that until Monday, we should have the server up and running until then with 1.4. No promises though!