Well, you also get HDR with the 4660. If you've seen the difference at 4K with and without HDR, that alone is worth the upgrade. If you still need the optical out, try to find a 4640 Ultra (2016 version). It does HDR and still has the optical out, as well as Dolby Digital + decoding to standard DD.
Dan Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.