Check your driver version, if it isn't working at all. Restart DS, perhaps, so that everything is clean. If that still fails you will need to look at reducing the memory footprint - unless you are making really large renders you can almost certainly reduce the texture sizes, there are free scripts for this or Scene Optimiser script in the Daz store. You may also remove maps that are not needed or simplify materials, and for distance figures you can probably lower the Render SubD level. However, a 4GB GPU - especially in the recent versions of Iray, which pass more code t the GPU for non-RTX cards - is going t be limiting.