Before discussing the third tetrad specifically, there is a very important point which we sometimes forget to stress. Every time you sit down to practice Anapanasati - every sitting and session - you must begin with step one, the experiencing of the long breath. It does not matter what step you were doing yesterday, today you must start again at the very beginning. Each session is brand new. From the long breath, move on to the short breath, and so on. Progress from one step to the next, completely fulfilling each step before moving on, until you come to the step where you left off last time. Each step depends upon the previous one. If you are unable to do the first step, then there is no possibility of you going on to further steps. Buddhadasa 

Before discussing the third tetrad specifically, there is a very important point which we sometimes forget to stress. Every time you sit down to practice Anapanasati - every sitting and session - you must begin with step one, the experiencing of the long breath. It does not matter what step you were doing yesterday, today you must start again at the very beginning. Each session is brand new. From the long breath, move on to the short breath, and so on. Progress from one step to the next, completely fulfilling each step before moving on, until you come to the step where you left off last time. Each step depends upon the previous one. If you are unable to do the first step, then there is no possibility of you going on to further steps. 

Buddhadasa 

Please understand that the realizations of these truths are interrelated. From seeing impermanence, we see unsatisfactoriness, see anatta, see sunnata; see tathata, and see idappaccayata (conditionality, the law of cause and effect), also. Each continues into the next. A complete realization of impermanence must include un­satisfactoriness, not-self, voidness, thusness, and the law of conditionality. When all of these are seen, then impermanence is seen completely in the most profound way. This is how we realize fully the impermanence of the sankhara.

http://www.dhammatalks.net/Books3/Bhikkhu_Buddhadasa_Anapanasati_Mindfulness_with_Breathing.htm

Another common problem is that some people cling to... the satipatthana of the Digha-nikaya (Long Discourses) which is not anything more than a long list ofnames, a lengthy catalogue of sets of dhammas. Although there are whole bunches of dhammas, no way of practice is given or explained there. This is what is generally taken to be satipatthana. Then it is adjusted and rearranged into these and those practices, which become new systems that are called satipatthana practices or meditation. 

Then, the followers of such techniques deny, or even despise, the Anapanasati approach, asserting that it is not satipatthana. In truth, Anapanasati is the heart of satipatthana, the heart of all four foundations of mindfulness. The 16 Steps is a straight-forward and clear practice, not just a list of names or dhammas like in the Mahasatipatthana Sutta (Digha-nikaya#22**).

Therefore, let us not fall into the misunderstanding that Anapanasati is not satipatthana, otherwise we might lose interest in it thinking that it is wrong. Unfortunately, this misunderstanding is common. Let us reiterate that Anapanasati is the heart of all four satipatthana in a form that can be readily practiced.        

So please understand correctly that whether we call it satipatthana or Anapanasati there are only four matters of importance:kaya, vedana, citta, and Dhamma. However, in the Mahasatipatthana Sutta there's no explanation of how to practice these four things. It gives only the names of dhammas and expands upon them. For example, the matter of kaya (body) is spread out over corpse meditations, sati-sampajanna in daily activities, the postures, and others more than can be remembered. It merely catalogues groups of dhammas under the four areas of study. 

The Anapanasati Sutta, on the other hand, shows how to practice the four foundations in a systematic progression that ends with emancipation from all dukkha. The sixteen steps work through the four foundations, each one developing upon the previous, and supporting the next. Practice all sixteen steps fully and the heart of the satipatthana arises perfectly. In short, the Satipatthana Suttas are only lists of names. The Anapanasati Sutta clearly shows how to practice the four foundations without anything extra or surplus. It does not mention unrelated matters. 

http://www.dhammatalks.net/Books3/Bhikkhu_Buddhadasa_Anapanasati_Mindfulness_with_Breathing.htm