I cut my losses and got the Steam version of GTA 4. Turns out that fixed the problem.
So now I've got the Sims 2 Ultimate Collection on Origin and GTA 4 on Steam.
This leaves me with just one last, but very minor problem.
I have a copy of GTA San Andreas on my computer as well, and the vehicle mods for GTA 4 and San Andreas both use the same format for their filenames. So now I can't tell the damn files apart!
Not to mention the fact that I'm so out of touch with GTA 4 that reinstalling the mods correctly will be like performing eye surgery with my feet.