source : https://www.nexusmods.com/gta4/mods/10
GTAIV 100% Savegame
If you want to change the slot rename the savegame file. Example SGTA400 fill the first slot, if you rename in SGTA05 it will fill 5th slot. Max slot is 12.
Make a backup of your savegames folder located in (XP):
C:Documents and SettingsUSERLocal SettingsApplication DataRockstar GamesGTA IV