Try this: right click on GTA4, select properties, then select launch options
copy and paste this into the field that comes up
-nomemrestrict -norestrictions
exactly like that,
the reason why its not letting you set anything is because gta4 only supported GPUS up to 2gb, so since yours is 3gb, it miscalculated how much ram was up for use and thinks you dont have enough, therefore enforcing this lock down on the graphics options,
that command line will bypass it