Oh BSSSSSSSSS, If was down 4,400 they could shut it off easily...hydraulic actuators operate at 3000 psi.
No, no BS and if you would do more research and read the thousands and thousands of bits of data on the internet you would know why.
But just in case you cannot read with comprehension, here is the reason: During the top kill operation they were afraid to overstress the upper casing, possibly causing a leak at a connection which could lead to a sub-surface blowout. Something nobody wants.
So, if all they had to do is put a valve on it would result in an immedate blowout below the sea floor and then it would be totally uncontrollable until the relief wells were completed. By doing it they way they are at least they can capture a majority of the oil.