I know how to solve, i had this too before. I've a notebook and you need to render it with the gta 4 video editor to your max. settings (notebool max. is 720p) after that, the rendered video will be saved into you Rockstar Games folder (Folder in your documents) then go to GTA IV and videos, then rendered (hope i tell the right location)
After that, download Windows Movie Maker if you dont have it.
Start movie maker and import the rendered video. Video import done? > publish or save the file as a HD video file (1080p) you can see it into the menu in movie maker. Then upload the exported (probably mp4 file) to youtube and you've fulscreen.