I have a Dell Inspiron 15 5565 laptop with FX 9800P processor and the R8 M445DX(512 mgb(primary) of R5 + 4GB of R7) combination for graphics. And I have been able to bend it to my will on most games. However, it seems like GTA 5 does not want to see the 4gb on this card. It only shows 1024 and that is causing textures not loading in the city and all sorts of stupid lags. I know it might depend on the version of the game too, as I have seen videos of people on youtube claiming to have the same specs while showing the full 4gb of VRAM on their settings. I have tried changing the direct x version to 10 and changing my driver and so far nothing seems to convince the game that I have 4gb of video memory(Or maybe I don't!). On Arkham Knight it was showing something like 500mg of VRAM. I don't get it. Any help will be appreciated.
Your laptop is physically wired to your integrated GPU, so GTA cannot see the R8.
Set GTAV to high performance the R8 GPU will be used, despite what GTA V says. Have a read of this document > How to Configure Laptop Switchable Graphics
You should note however that your specs fall below the minimum requirements to play the game, so I wouldn't expect great performance.