How I Got AvantGo 4.0 Working
No guarantees that this will work for anyone else.
My problems were that when I installed AvantGo 4.0, it didn't put the .prc files in the install folder. It also removed the conduit from HotSync manager for some odd reason.
Here's how I fixed it and got 4.0 working:
1. I re-installed AvantGo 3.3
2. I searched the registry (regedit.exe) and exported all entries releated to the AvantGo conduit (did a search for "Mobile Link")
3. I installed AvantGo 4.0
4. I went to the program files/AvantGo folder and added avantgo.prc, libmal.prc, and MobileLink.prc to the HotSync install tool (just double clicked them)
5. I imported all registry entries from step 2
6. I HotSync'd to get the new files on my Visor and HotSync'd again to get the updated pages.
Hopefully this will help someone.