The brick wall at 4k rpms is your car going into open loop and maxing out the injectors. When your duty cycle hits 100% from the PCM it cuts spark. Not fuel. Everyone calls this fuel cut, but it is actually ignition cut. Anyone who disagrees with me can put a scope on pins 74, 75, 100, 101 and pull their WGA line and watch what happens when they overboost.
As soon as ignition is cut the MAF isn't reading nearly as much air and the car can continue on its merry way. There is actually a little spike in boost just before the wastegate opens and can cause the car to reach spark cut. The first couple hundred rpm of open loop are also programmed to be excessively rich to safeguard against negative short term fuel trims. I've seen my car go into the 9s completely stock.
If your car is completely stock the most likely reason you would hit cut is because your WGA is bad. Take the vacuum line from your WGA and follow it back and pull it from the T and blow in it. If you can blow through your WGA, it needs to be replaced. The diaphragm is rubber and over time will lose its seal with the housing.
I had this happen around 50k miles and replaced it with the ATP one from PG and it's been solid for the past 26k miles.