Hello,
Depending on the type of game, often the graphics card determines what settings you are able to play a game at. My philosophy is that generally pre-built computers with mid to high end graphics card will often have suitably powerful processor and RAM.
Because The Sims 4 is simulating and doing a number of calculations, a processor with more cores is very useful for the higher settings. I would say for this game, the processor and graphics card are of equal importance for performance.
I hope this helps and keep simming,
Cody