You might be interested in reading the commentary to the Satipatthana Sutta here:
Cattaro Satipatthana = "The Four Arousings of Mindfulness." Four in relation to classes of objects of mindfulness.
Why did the Buddha teach just Four Arousings of Mindfulness and neither more nor less? By way of what was suitable for those capable of being trained.
In regard to the pair of the dull-witted and the keen-witted minds among tamable persons of the craving type and the theorizing type, pursuing the path of quietude [samatha] or that of insight [vipassana] in the practice of meditation, the following is stated: For the dull-witted man of craving type the Arousing of Mindfulness through the contemplation of the gross physical body is the Path to Purity; for the keen-witted of this type, the subtle subject of meditation on the feeling. And for the dull-witted man of the theorizing type the Path to Purity is the Arousing of Mindfulness through a subject not too full of distinctions, namely, consciousness [citta]; for the keen-witted of this type, the subject which teems with distinctions, namely the contemplation on things of the mind — mental objects [dhammanupassana].
For the dull-witted man, pursuing quietude, the First Arousing of Mindfulness, body-contemplation, is the Path to Purity, by reason of the feasibility of getting at the mental reflex; for the keen-witted of this type, because he does not continue to stay in the coarse, the second Arousing of Mindfulness, the contemplation on feeling, is the Path to Purity.
And for the dull-witted man pursuing the path of insight, the subject of meditation without many distinctions, the contemplation on consciousness, is the Path to Purity; and for the keen-witted of this type the contemplation on mental objects which is full of distinctions.
Or it may be said that these Four Arousings of Mindfulness are taught for casting out the illusions [vipallasa] concerning beauty, pleasure, permanence, and an ego.
The body is ugly. There are people led astray by the illusion that it is a thing of beauty. In order to show such people the ugliness of the body and to make them give up their wrong idea, the First Arousing of Mindfulness is taught.
Feeling is suffering. There are people subject to the illusion that it gives pleasure. In order to show such people the painfulness of feeling and to make them give up their wrong idea, the Second Arousing of Mindfulness is taught.
Consciousness is impermanent. There are people who, owing to an illusion, believe that it is permanent. To show them the impermanence of consciousness and to wean them of their wrong belief, the Third Arousing of Mindfulness is taught.
Mental objects are insubstantial, are soulless, and possess no entity. There are people who believe by reason of an illusion that these mental things are substantial, endowed with an abiding core, or a soul, or that they form part of a soul, an ego or some substance that abides. To convince such errant folk of the fact of the soullessness or the insubstantiality of mental things and to destroy the illusion which clouds their minds, the Fourth Arousing of Mindfulness is taught.
[I believe italics indicate the sub-commentary:}
Drawing distinctions, it is said: Body and feeling are the cause of zest [assadassa karana]. For the rejection of that zest of body, by the dull-witted [manda] man of the craving type [tanhacarita], the seeing [dassana] of the ugly [asubha] in the body, the coarse object [olarika arammana], which is the basis of craving [tanha vatthu], is convenient. To that type of man the contemplation on corporeality, the First Arousing of Mindfulness, is the Path to Purity [Visuddhi Magga]. For the abandoning of that zest, by the keen-witted [tikha] man of the craving type, the seeing of suffering in feeling, the subtle object [sukhuma arammana], which is the basis of craving, is convenient, and for him the contemplation on feeling, the Second Arousing of Mindfulness, is the Path to Purity.
For the dull-witted man of the theorizing type [ditthi carita] it is convenient to see consciousness [citta] in the fairly simple way it is set forth in this discourse, by way of impermanence [aniccata], and by way of such divisions as mind-with-lust [saragadi vasena], in order to reject the notion of permanence [nicca sañña] in regard to consciousness. Consciousness is a special condition [visesa karana] for the wrong view due to a basic belief in permanence [niccanti abhinivesa vatthutaya ditthiya]. The contemplation on consciousness, the Third Arousing of Mindfulness, is the Path to Purity of this type of man.
For the keen-witted man of the theorizing type it is convenient to see mental objects or things [dhamma], according to the manifold way set forth in this discourse, by way of perception, sense-impression and so forth [nivaranadi vasena], in order to reject the notion of a soul [atta sañña] in regard to mental things. Mental things are special conditions for the wrong view due to a basic belief in a soul [attanti abhinivesa vatthutaya ditthiya]. For this type of man the contemplation on mental objects, the Fourth Arousing of Mindfulness, is the Path to Purity.
Consciousness and mental objects constitute the outstanding conditions of theorizing. Consciousness is such a condition because it is a decisive factor in the belief in permanence. Mental objects are such conditions because these are decisive factors in the belief in a soul.
Consciousness and mental objects are decisive factors of craving as well as of theorizing. And body and feeling are decisive factors of theorizing as well as of craving. Yet to point out that which is stronger in body and feeling, namely, craving, and that which is stronger in consciousness and mental objects, namely, theorizing, distinctions have been drawn.