Which plant genus contain two to four species, Greyia or Calycanthus?
Greyia:  It contains three species: Calycanthus: Calycanthus (sweetshrub) is a genus of flowering plants in the family Calycanthaceae, endemic to North America. Calycanthus:  The genus includes two to four species depending on taxonomic interpretation; two are accepted by the "Flora of North America".
Calycanthus