Increasing your machine's RAM (that you see in the control panel under system) is completely fine. The graphics card doesnt matter. If you want more Video Ram or Vram that is on your graphics card (in your case your gtx 750ti) you will have to buy a new graphics card with more Vram because it is imposible to add Vram.
Do you mean you have 4gb ram in your system? If thats the case yeah thats fine-your system's ram will not have any bearing on the potential compatibility of the gpu.
If you are asking if you can increase the vRam of the graphics card there is no way to do it other than buying a higher end card, but if your asking if 4Gb of "standard" ram will be compatible with the card, then yes.
Increasing your machine's RAM (that you see in the control panel under system) is completely fine. The graphics card doesnt matter. If you want more Video Ram or Vram that is on your graphics card (in your case your gtx 750ti) you will have to buy a new graphics card with more Vram because it is imposible to add Vram.