All current Roku players support 720p and 1080p. Some also support 2160p (4K). All current 4K players support HDR as well.
The only real difference is features offered on the different models. The Ultra is the only one with a wired network connection. Some only have single band (2.4 GHz) WiFi, while some are dual band and support 802.11ac. Some only come with IR (infrared) remotes, while others use a WiFi Direct version which doesn't require line of sight between the remote and the Roku.
Dan
Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), 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.
