So, I found this pretty old (maybe 3-4 yrs) Liquid Foundation by Mac. It seemed to have a normal consistency but when I put it on it smelled kind of funky/old. I washed it off immediately, but do you think I will get an infection??

Have mercy, Girl!
Relax, I've never heard of anybody actually getting an infection from face make-up before. If anything, you would of gotten an allergic reaction to it by feeling burning and or itching on your skin, with the possibility of a rash. But that's not serious, just annoying. It probably is too old, so toss it in the trash. Take a deep breath and relax. I bet you don't even need that heavy make-up anyway. I bet you looked very beautiful with a "just washed face" than with that thick make-up on.
Stay beautiful and relax!

DONT ever use old foundation!! its bad for ur skin! u wont get an infection but please for the sake of ur skin dont use it. buy a new foundation and replace every 6 months.

I'm not too sure about infections, but it's a high chance you'll get a breakout from it. DON'T use expired make-up. Bad stuff.

no not if you washed it off right away
don't use it though. it'll probably break you out