I have a grade 4 cerebral avm. My feet and now my thumb have become both numb and painful. What is happening?

Shunting blood. The avm is probably shunting blood from that region of your brain. Grade 4 avms are difficult to treat, morbidity and mortality from surgical resection is in the range of 20%. Other modalities that can be used to treat an avm include embolization and radiation.
Not sure. Depending on the location and size of your avm, your symptoms may or may not be associated. See your neurosurgeon if you are experiencing any symptom changes. Let him decide if it's a big deal or not!