I have been having a lot of random sharp pains in my lower abdomen that shoots down my left leg. This happens only when I am sitting down and staying still. Also I just recently got some lower back pain that is constant... doesn't go away. It's not painful but it's enough to notice. Should I go to the ER? Or wait another 4 days until I go to my OB GYN?

We know nothing about you other than that you are pregnant! How far along, history, general health, weight, edema, blood pressure, on and on.
Call your OB office on Monday morning if you get worse.
ER if something obviously serious (fever, high blood pressure) but check in with your OB if you can, or the doctor covering.
Low back pain goes with pregnancy - very common, and the nerves off the lower spine go down the legs.
It's tempting to tell you to just wait 4 days, but again, we know nothing about you.

(As someone with chronic lower back pain and related pain from those nerves, I can tell you that if I sit, stand, or lie down too long I hurt a lot more, so I have to keep changing what I'm doing. Sitting is the worst, but I have developed many techniques for how to sit, lean back, rotate my torso, stretch my legs out, and make sure that my weight is equal across my butt and feet. Feet must be flat on the floor if on a chair, or your back sways inward. Don't lean forward to peer at your computer screen! Sit straight!
Get up and putter around for 5 minutes every 20 minutes, at a minimum. On a couch, put pillows behind you and put your feet up. Go on some pregnancy sites and research all the ways to avoid back pain. And eat enough fiber, and drink fluids, because every bit of fullness in your bowel will be noticeable.)