The water removed has to go somewhere. There is mo point taking too much out too quickly, as that will cause severe flood damage downstream, which is what they are trying to prevent.
The threat of the dam bursting has now past. The four months is the time it will take to reduce the water level in order to do a full inspection and make effective repairs.
The reservoir has a function, which is to supply the Peak Forest Canal with water. So they don’t want to empty it, they want to reduce it to a safer level.