So you have the Ultra 4640. Yes, there are numerous changes to the latest version (4800/4802).
First, the items removed.
There's no longer an optical output, and there's no SD card slot. The optical out is only an issue if you are using an older AVR that doesn't have HDMI inputs. The loss of the SD card is a non-issue, because the onboard channel storage has been enlarged 4x.
Also be aware that Walmart sells an Ultra 4801 which is lacking a USB port. This too might not be an issue for your use.
The 4800/4802 increases operational memory to 2 GB, and channel storage memory to 4 GB.
It has a completely new processor, which is more powerful, so performance is improved.
It now support Dolby Vision, in addition to HDR10, HDR10+ and HLG.
It supports Dolby AC-4 audio. This isn't too important at the moment, but the new ATSC 3.0 broadcast standard will be using that audio codec, and it's possible that online streaming providers may also be moving to it in the future. AC-4 offers significant improvements over AC-3/Dolby Digital, including more potential audio channels and higher quality sound, all with a lower bitrate.
It also includes a Dolby audio encoder, so it can convert AC-4 to AC-3 for devices that can't play AC-4.
It has improved WiFi, now offering MIMO support.
And if you misplace your remote, you can trigger it to make a pinging sound to help locate it.
Not certain if I've missed anything, but these are the most notable items.
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Well, I've bumped the button a couple of times and the remote always made itself known.
Well, maybe the Roku Gods hold you in better regard.
Mine has never worked. Trying to find it with the button on the Ultra or use iOs app does not work. The only way to trigger it is to use the remote that you're trying to find and go to the find the remote in settings.
I'm not alone either as there are multiple threads complaining about the feature not working. Roku acknowledges it doesn't work and promises after each software update it will. I'm at 11.0.0. Hasn't worked yet.
I'm a little spicy about it as I had to tear the room apart to find the darn remote. I just wish it wasn't advertised as a feature when it fails for so many.
1) Dolby decoder: Decodes EAC3 (DD+) to AC3 (DD) for output over HDMI (for compatibility with legacy DD-only TVs/AVRs/SBs - most streaming services exclusively use EAC3/DD+ - 4640 includes this too, posted to emphasize functionality maintained)
2) AV1 codec support: YT/YTTV/NF all support AV1 for greater compression efficiency over VP9, and other services will likely migrate to it (at least partially) as wellin the near future.
3) Bluetooth Support: Currently limited (by OS 11 and earlier) to using your Ultra/connected system as an an output device (paired with a phone/tablet/etc) - with OS 11.5 (rolling out now) BlueTooth Private Listening is enabled (pair BT headset/speakers directly to the Ultra, bypassing the need for the remote or a phone/tablet).