Not taking proper RAM! Too much lag

I got 4GB of Nvidia MX150 card but the game( GTA IV ) seems to not be taking more than 512 MB of it and RAM is 16GB with Intel -i7 8th Gen, no clue why, though GTA V (which is not on lutris) seems to work perfect on this card.

Here is some technical info about the Nvidia MX150.:

Regarding your question, there are several factors which could be of influence. For instance, the resolution you are running directly impacts the amount of ram needed. Could it be that you’ve toned down the detail level to make the game playable? Could it be that the game simply doesn’t need more ram at this detail level?

Bit hard to guess at my side but that is where I would begin investigating.

Okidoki so this is actually a pretty well known issue with GTA:IV, it isn’t exactly great on PC. Try the following under Game Options, Arguments: -nomemrestrict -norestrictions -percentvidmem 100 -availablevidmem 4096

Good luck, this game is still a mess.