The setting is a tick box in the output style, citation, numbered setting (the first one which is ticked in this example and should produced the [1-4] I think you want). However there are some “output styles” out there from an earlier version of endnote that changes in the numbering aren’t applied as expected. The safest thing to do in that case is to jetison the version of the style you are using and download a fresh one from the Help, Web styles finder and try adjusting that to your requirements.