what exactly is a RV that is "4 seasons ready"?

Typically a 4 season unit will have no problem dealing with minus 10 C temperatures. Some are considerably better than that. Most are designed so that you must use your furnace (as opposed to using portable heaters) as the furnace ducts air to tank areas. The dump valves are also enclosed.

Yes its a selling feature. We wouldn't have bought our coach if it wasn't "four season". Ours seems liveable down to about minus 20 C.