Hi wolf and welcome to SF.
Normally those settings are either made in the game itself or like in this case executed as a switch in the Target line of your Shortcut. But GTA4 apparently handles that a little different.
Try it, a little help for some low end pcs
-nomemrestrict -novblank -renderquality 0 -shadowdensity 0 -texturequality 0 -viewdistance 1 -detailquality 1 -windowed -percentvidmem 100 -noprecache -dx9
1. Write down this text in a single line in a txt file,
2. rename it to commandline.txt
3. drop in " \Rockstar Games\Grand Theft Auto IV" folder
4. Run the game
If you really only need the windowed part, just leave tout he other "optimizations" so your commandline.txt holds only -windowed