WELCOME ABOARD!
SH3 is actually more graphics intensive in some areas than SH4. For instance, I've done several upgrades to my computer, and SH4 now runs very well, but with SH3 my game always lags when I look at heavy fire and smoke, and I can't even run the super-heavy smoke mods without crashing.
If you can't improve your graphics somehow I think you're going to have to live with it. I'm hoping someone will tell me I'm wrong and tell you how to fix it, but until then that's all I can think of.