XXX vs XXXX

XXX

adj
  • Designating extreme or hardcore pornography. 

  • Extra strong; very high quality. 

symbol
  • An indicator of hazard or poison. 

  • Traditional brewer's mark for barrels of extra strong beer—extended to other alcoholic beverages and other products having an extra strong dimension (varying from laundry soap to pornography). 

verb
  • To delete; to cross out, especially to conceal or suggest vulgar language. 

noun
  • Triple X syndrome. 

XXXX

adj
  • X-rated, pornographic 

name
  • a fuck / damn. 

  • Tetrasomy X, a rare chromosomal disorder affecting females, caused by the presence of four X chromosomes instead of two present in the usual human karyotype. 

How often have the words XXX and XXXX occurred in a corpus of books? (source: Google Ngram Viewer )