If one cultivates the four foundations of mindfulness by observing the breath, will one fall into inert emptiness?

The result of the contemplative practice (vipaśyanā-caryā) of the four foundations of mindfulness is the elimination of self-view (ātma-dṛṣṭi), the attainment of the fruit of liberation, and the severing of afflictions to gain liberation. Since every person has afflictions and self-view, everyone is an ordinary being (pṛthagjana), and therefore needs to eliminate self-view—and eliminating self-view must begin with the contemplation of breathing in the four foundations of mindfulness. Ordinary beings all have a self, have all kinds of minds, and have all kinds of afflictions; that is precisely why they need to cultivate the state of no-self and no afflictions. Practice should begin step by step from the foundation, starting with the contemplation of breathing, progressing from the shallow to the deep; only then can one realize no-self and attain a mind that is empty and free of afflictions. 

For an ordinary being, no matter what kind of emptiness it is, as long as you can become empty even a little bit, that is quite good already. Try your best to become empty, to let go—do not fear inert emptiness or annihilatory emptiness (uccheda-śūnyatā), because ordinary beings simply cannot achieve inert emptiness or annihilatory emptiness anyway. So empty yourself as much as possible; the emptier, the better. Forget all the knowledge and understanding you have previously acquired, hollow out your mind, become like an infant, and start learning again bit by bit. All the knowledge and understanding learned before are like tangled vines (kudzu), crisscrossing and lodged in the mind—neither up nor down, blocking it uncomfortably. Now empty them all out, make the mind clean and pure, and then fill it anew with pure dharmas. What was learned before but could not be digested may in some cases turn into an illness. If you can empty it out, the illness is cured; if you cannot empty it, you remain ill. Or you must find your own way to treat the disease—others can only offer suggestions; the only one who can cure it is yourself. 

The more theory you come into contact with, the harder it is to digest, and it all becomes lodged in the mind, to the point that the medicine turns into an illness. If your capacity for digestion and absorption is not good, you should clear away some of the deeper theories and instead practice and operate more on dharma teachings that are easy to digest and absorb. However good a theory may be, if you cannot digest it, you should set it aside for the time being and begin cultivating from the foundational dharma teachings—do not aim too high and reach too far.