Generally the "weakest" function is considered to be the 5th function (suggestible). Not 4th (vulnerable).

This is because the 4th function, at least in the traditional Model A, is consciously accessible. By actively going against their inclination to avoid this function, and exerting mental energy, people can sometimes achieve some pretty good results with their 4th function. This is more than can be said of the 5th function, which is usually depicted as helplessly incompetent and no amount of effort or introspection can force it to improve.