The Four Dharmadhatu is a philosophical concept propagated by Master Tu-shun

Dushun

Dushun was the First Patriarch in Hua-yen school of Chinese Buddhism. The Indian Avatamsaka Sutra is its central scripture. This school originated in China. It is known as Kegon in Japan. The Avatamsaka's seminal chapter once circulated separately and is known as The Gandavyhua Sutra. Each...

 (Chinese: 杜順; 557-640 CE). It builds upon and is a variant of the Dharmadhatu

Dharmadhatu

Dharmadhatu may be defined as the 'dimension', 'realm' or 'sphere' of Dharma and denotes the collective 'one-taste' dimension of Dharmata.-Nomenclature, orthography and etymology:...

 doctrine. Tu-shun is the founder of Hua-yan (Chinese: 華嚴) school. The Four Dharmadhatu were outlined in Tu-shun's treatise which has been rendered into English as 'On the Meditation of Dharmadhātu'. The Four Dharmadhatu are:

• The Dharmadhātu of 'Shih' (Chinese: 事法界; "shi fajie"). 'Shih' is a rendering of the character 事 which holds the semantic field
  Semantic field
  A semantic field is a technical term in the discipline of linguistics to describe a set of words grouped by meaning in a certain way. The term is also used in other academic disciplines, such as anthropology and computational semiotics.-Definition and usage:...
  
  : "matter", "phenomenon", "event". It may be understood as the 'realm' (Sanskrit: dhātu) of all matters and phenomena.
• The Dharmadhātu of 'Li'(Chinese: 理法界; "li fajie"). 'Li' is a rendering of the character 理 which holds the semantic field: "principle", "law", "noumenon
  Noumenon
  The noumenon is a posited object or event that is known without the use of the senses.The term is generally used in contrast with, or in relation to "phenomenon", which refers to anything that appears to, or is an object of, the senses...
  
  ". This 'realm' (Sanskrit: dhātu) may be understood as that of principles. It has been referred to as "the realm of the one principle". The "one principle" being qualified as śūnyatā (Sanskrit).
• The Dharmadhātu of Non-obstruction of 'Li' against 'Shih' (Chinese: 理事無礙法界; "lishi wuai fajie"). This 'realm' (Sanskrit: dhātu) has been rendered into English as "the realm of non-obstruction between principle and phenomena".
• The Dharmadhātu of the Non-obstruction of 'Shih' and 'Shih' (Chinese: 事事無礙法界; "shishi wuai fajie"). This 'realm' (Sanskrit: dhātu) has been rendered into English as "the realm of non-obstruction between phenomena".