Will practicing the four establishments of mindfulness and contemplating the breath fall into inert emptiness?

The result of the contemplative practice (vipaśyanā-caryā) of the four foundations of mindfulness is the eradication of the view of self (satkāya-dṛṣṭi-prahāṇa), the attainment of the fruit of liberation, and the elimination of afflictions to gain liberation. Since every person has afflictions and has the view of self, everyone is an ordinary being (pṛthagjana), and therefore needs to eradicate the view of self—and eradicating the view of self 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 selflessness and freedom from 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 selflessness, and only then can the mind be empty and free of afflictions. 

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

The more theory one comes into contact with, the harder it is to digest; it all gets stuck in the mind, to the point that the medicine becomes the illness. If your capacity to digest and absorb is not good, you should clear away some of the deeper theories and practice more hands-on with dharma teachings that are easy to digest and absorb. No matter how good a theory is, if it cannot be digested, it should be set aside for the time being; begin practicing from the foundational dharma teachings, and do not aim too high.