This is a somewhat complex question. It's true in WFM 4/AES 8.0 that the managed catalog is the same database as the WFM database.
Here's what I would recommend trying:
1. Backup all of your databases (including the AES 7.5 'db' folder)
2. Run the WFM 4.0 installer to upgrade 3.5 -> 4.0. The will upgrade the database as well to the new schema.
3. Connect to your AS 7.5 database using the AdminStudio 8.0 IDE. This will prompt for upgrade as well.
4. Using SQL Server DTS, merge everything into one database.
For using the WFM Database as the managed catalog:
--Merge everything but the AMS_* tables from the AS 8 database into the WFM database.
For using the AS 8 database as the managed catalog:
--Merge the AMS_* tables into the AS database
--Change the 2 entries in WFM 4's web.config file to point to the AS database
5. Lastly, use the AES7Migrator utility that gets installed with WFM 4 to migrate all your existing roles and such from AES 7.5. This actually has some documentation here:
HOWTO: Migrate Custom Users and Roles in AES 7.x to AdminStudio 8:
http://support.installshield.com/kb/view.asp?articleid=Q112624