You cannot copy/move individual save slots on the Xbox 360 dashboard. The console treats each profile's save data as a monolithic container tied to the signing-in User ID (XUID).
%LOCALAPPDATA%\Rockstar Games\GTA IV\savegames\. Copy SGTA40000.bmp + savegame.dat files between slots manually. Supports trainers/mods.Warning: Tools like Modio, Horizon, 360 Content Manager to resign/rehash saves for different profiles **violate Xbox Live ToS** and risk **permanent console/device ban**.