A:Answer The 43S405 is a member of our 4-Series, while the 43S517 is a member of our 5-Series. Both series include a built in Roku operating system, 4K UHD, 120hz CMI refresh rate, and HDR Support. The 5-Series goes a bit further and has additional features such as Dolby Vision and wide color gamut. Hope this helps! Thank you, TCL Customer Support