Fix chapter 4 modules
Fix chapter 4 modules

ruisalgado/purescript-book,ascjones/purescript-book,ruisalgado/purescript-book,beckyconning/purescript-by-example,rick68/purescript-book,paf31/purescript-book,ascjones/purescript-book,mikemcgowan/purescript-book,shybyte/purescript-book,endeavour/purescript-book,shybyte/purescript-book,ericssmith/purescript-book,rick68/purescript-book,paf31/purescript-book,beckyconning/purescript-by-example,mikemcgowan/purescript-book,endeavour/purescript-book,ericssmith/purescript-book