There are four types of master pages in GN4: Left, Right, Double (that you can select in the right-click context menu on the Editions main tab in Fred4) and Generic (not assigned to a type).

Every edition has to have one and only one master page assigned to Left, Right and Double type, and all other master pages have to be Generic.

Notes about naming generic master for easier renumbering

If you create generic master that are supposed to go in pairs (e.g. News generic master for odd pages and for even pages), you must specify such master names in the way the contain -L or -Left, and -R or -Right, e.g. News -Left or News -L, News -Right or News -R. This provides the maintaining of assigned generic masters when you insert a single page between existing pages.