What is the 4th ventricle?
Cerebro-Spinal Fluid (CSF) is produced in the right and left ventricles then flows into a 3rd ventricle (connecting the two) in the mid-upper brain then through the 4th ventricle and down around the spinal column, circulating back up the column to be reabsorbed around the brain. If the 4th ventricle was blocked, you would develop hydrocephalus and need to have a shunt placed to direct the flow or CSF out of your brain and usually down into the abdomon. The 4th ventricle can be compressed or flattened without obstructing flow. It is only when flow is totally obstructed that it becomes a real issue.
If you wake up with a massive headache after surgery, it is because the CSF was released when your tumor was removed. Even though the brain can't live without oxygen, it likes its oxygen comsumption via the bloodstream. When the air hits the brain, then brain becomes very unhappy. (Spinal Headache is an example of this.) The headache will come and go until sufficient CSF has been produced to circulate through the system and cover the brain again.
Hope I answered some of the question. Like battyprincess, my memory isn't what it used to be........