What are the 4 wheels and 9 doors in Snp 1.29? What are the four wheels and nine doors mentioned in the sutta below?

Why is it "born from a bog"?

What is the meaning of the sutta?

From the Four Wheels Sutta (Sutta Nipata 1.29):


  “Having four wheels and nine doors,
  Filled up and bound with greed,
  Born from a bog, O great hero!
  How does one escape from it?”    
  
  “Having cut the thong and the strap,
  Having cut off evil desire and greed,
  Having drawn out craving with its root:
  Thus one escapes from it.”
four wheels and nine doors

It means 4 limbs(?), 9 body openings (7 head, 2 lower) -- so, a human/being in sensual world. By cutting off kama-chanda, one leaves the sensual realm (human would &amp; other sugati realms, is bound to higher heavens). As quick perceived. It seems not related to direct Nibbana (although "with its roots" is mentioned, but the wish was related to the dirty sensual realm).

See also AN 2.35: Samacitta Sutta — Minds in Tune

Further investigations in regard of "cakka", wheel:

More common is Chakra, for the "wheels" and might be also denote old indian models of "life-energy", usualy 5 or 7 charkras, which leads to


  The esoteric traditions in Buddhism generally teach four chakras.[3] In some early Buddhist sources, these chakras are identified as: manipura (navel), anahata (heart), vishuddha (throat) and ushnisha kamala (crown). en.m.wikipedia.org/wiki/Chakra


but as still not really proper seeming, a Pali research gives a better answer:


  catucakka an instr. of four, a lucky tetrad, a fourwheeler of the body as expressing itself in the four kinds of deportment, iriyāpathas A ii.32; S i.16, 63 (catucakkaŋ). In this sense generalized as a happy state, consisting of "4 blessings": paṭirūpadesa -- vāsa, sappurisûpassaya, atta -- sammāpaṇidhi, pubbe -- kata -- puññatā, https://dsalsrv04.uchicago.edu/cgi-bin/app/pali_query.py?qs=Cakka&amp;searchhws=yes


(sugati loka, human with certain merits)

bog: refers to birth in a womb, phlegm... swamp. The questioner obiviously aware of asubha of body, form.