The 4th tetrad (contemplation of the Dhamma) seems to mean experiencing/contemplating anicca/fading away/cessation/relinquishment of every in-breath and out-breath, instead of things other than breath.If you are interested in the details, I would recommend:- Anapanasati chapter in Patisambhidamagga <a href="http://bps.lk/olib/bp/bp502s.pdf">http://bps.lk/olib/bp/bp502s.pdf</a> http://dhamma.ru/forum/index.php?topic= ... 93#msg9193 ;- Anapanasa