I had exactly the same problem after I upgraded to Fusion 4...not able to access the shared folders.  I did several things and afterwards the problem was resolved.  First, since I had upgraded the virtual machine, I deleted my older snapshots and reclaimed the space on the virtual machine.  I turned off the shared folders.  I then uninstalled the VMware Tools and then reinstalled them.  Then I turned the shared folders back on.  Now, it works as well as it used to.