Hello there is a glitch with GTA 4 so that it doesnt reconigze graphics cards over 3GB of ram.here is a link to Rock* site with answers as I cant remember how I fixed my 980 ti but I did. also steam may have some safty precaustions you can add in properties.
https://support.rockstargames.com/hc/en-us/articles/200144856-Users-with-3GB-of-RAM-experiencing-frequent-crashes-in-GTA-IV