How many pages does a book leaf have?
A quick Google search revealed that


  Very generally, "leaves" refers to the pages of a book, as in the common phrase, "loose-leaf pages."
  
  A leaf is a single sheet bound in a book, and a leaf has two pages. (Source: Biblio)


A folded grouping of leaves is a "gathering", so the left image under "leaf with 4 pages" in your question is actually a gathering of 4 leaves (8 pages).  The right hand image is a gathering of 2 leaves (4 pages)

Note that if you tear one leaf out of a bound book, as books are more commonly bound in gatherings, often one other leaf will also become loose, and maybe fall out — especially if the leaf torn out is in the centre of a gathering. But that is a separate leaf, not the other half of the same.