There seems to be a general problem with your installation(s), maybe already with steam itself, because steam sets your paths and installs for you. If there is an error it is most likely steams fault.
A possible try would be this:
Make Backups of your games and reinstall steam fully. Then download only GTAIV new and do the downgrade.
If this works then (as it should), you can of course try to downgrade your old GTAIV-Backup as well, as you already have a downgraded GTAIV (backup this new one locally).
If this second downgrade works now, on your old
GTAIV-installation, you can be sure that steam had a problem.
English is not my native language, so be patient with me.