What is the difference between Roku Ultra 2020 and Roku Ultra 2021??
Assuming you are comparing your 4660 to the latest 4800, there's quite a few differences.
First, the 4800 has a faster processor. It's noticeably faster than my 4640 and 4670 Ultras. It also has much more channel storage and operational memory. Put together, the 4800 is much faster than older versions. It has built-in support for the AC-4 audio codec, which is beginning to be used with some Internet streaming sites. It adds MIMO to the WiFi radio for improved wireless performance. The final major improvement is support for Dolby Vision.
In the loss column, the 4800 does not have an SD cart slot for adding channel storage space. Since the 4800 has so much channel storage now, it's likely a non-issue.
Yes, your 4660s will get OS 10.0. I can't see any benefit to upgrade them to the 4670, but moving to the 4800 would be noticeable. To be honest, unless Dolby Vision would be a major draw for you, I don't think I'd bother moving from the 4660 to the 4800. Yes, it performs better, but I still use my Ultra 4640 and have zero issues. Since my LG is a DV display, it made sense to add the 4800 to it. The 4670 is connected to a small 1080 TV, and the 4640 is in my suitcase for when I travel.
Don't pay more than $99 for any Roku Ultra. That's the price on the Roku web site, and anywhere it's being sold higher is scalping.
The only difference between the Ultra 4800 and 4802 that's listed in the Wiki is the 4802 doesn't have a USB port. Same processor, same memory, same Fast Ethernet connection.
Buffering can come from two reasons: slow network and/or the media has too high a bitrate for the processor. If you're streaming from the Internet, neither of those should be an issue, as it's rare to find anything on the Internet that exceeds 20 Mbps or so. The exception is streaming from a home media server, such as Plex, Emby, or a DLNA server on your home network. With UHD video rips, it's easy to exceed the Fast Ethernet speed of a wired connection with any Roku that has Ethernet.
My solution for that is using WiFi, but it has to be 802.11ac on 5 GHz to be faster than the wired connection. I have tested my WiFi on my Ultra 4800, and the Roku has a max speed of about 220 Mbps on a strong 5 GHz WiFi. I've also tested the Ultra with some fixed bandwidth video files, playing from the USB port so no network bottleneck. The Ultra begins occasional buffering at around 180 Mbps, so that's about the limit of any existing Roku device. My UHD rips average around 80 Mbps, with peaks around 150 Mbps, so the Roku can handle them as long as my server transcodes the audio to a supported codec like AC3.
