Actually 8Gig is plenty for TS4, the x64 version never addresses too much more than 4gig if you ever looked in task manager, or unless your heavily multitasking with HD video at the same time.
Your CPU vs amount of cores you got, and a decent GPU that can keep up with you CPU demands and won't bottleneck is where your performance is concerned.
I got my game installed on dozens of machines, some 8Gig, 16gig, and a few 32gig, if I was comparing these, it be like comparing the size of my fuel tank, not engine performance.