Instructions
Insert a memory card or USB drive into the Xbox 360 and navigate to "Profile Settings" menu on the Settings screen of the Dashboard.
Select "Memory," then "Hard Drive," then select the GTA IV saves one by one and select "Copy" for each one. Copy to the USB drive or memory card.
Delete the profile that the game save came from on the Xbox 360, then log into the profile you want these files to be associated with. Insert the USB drive or memory card, then select the memory card or USB drive. Select the GTA IV saves and select "Copy." When you start GTA IV, these saves will load normally instead of giving you an "Unknown Profile" error.