4’ is national and international code; however, we generally recommend no more than 6’ if you want an easy time tensioning the cable.  It is also recommended to only use one straight end stop per cable run.  You should have at least one tensioning assembly (turnbuckle) on each run.  If you do plan to space your posts that far, I would suggest using 2 tensioner assemblies, one on each side of the cable and no end stop assembly.