What are the unthinkable (acinteyya) dhamma? I have heard there are 4 unthinkable (acinteyya) concepts in Buddhism. Not Acintya.

The four imponderables are identified in the Acintita Sutta, Anguttara Nikaya 4.77, as follows:[5]


  
  The Buddha-range of the Buddhas [i.e., the range of powers a Buddha
  develops as a result of becoming a Buddha];  
  The jhana-range of one absorbed in jhana [i.e., the range of powers that one may obtain while absorbed in jhana];  
  The [precise working out of the] results of kamma;  
  Speculation about [the origin, etc., of] the cosmos is an imponderable that is not to be speculated about.
  


Why It says Unthinkable?
The reason is given in the sutta itself. Pondering on these brings madness and frustration for someone with an undeveloped mind. 


  "These are the four unconjecturables that are not to be conjectured
  about, that would bring madness &amp; vexation to anyone who conjectured
  about them."
  http://www.accesstoinsight.org/tipitaka/an/an04/an04.077.than.html#fnt-1