This is all an edit.

I was looking for a working URC set of IR codes for the Apple 4K and I found that some of the codes in one set, and some in another set, seemed to work. Investigating this, I found the following:

EDIT: I spoke with URC. Their solution is to follow the weird-ass process that Apple TV provides. I'll call it reverse learning because it's backwards from the IR code learning or assigning process that we've used since the late 80s.

URC has a set of IR codes that don't do anything. You put those into your remote. Then, using the native Apple TV remote, you navigate to the section of Apple TV named "Add a remote." You then teach the Apple TV which commands you want to send to it for each function. Yes, this is backwards.

Taking PLAY as an example, you locate PLAY in the Apple TV list, then point your remote at the Apple TV and hit the PLAY button of that URC code set. You then associate that command with the function PLAY on the Apple TV. It has now learned that when it sees that code, that code means "PLAY." So it goes into PLAY. Yes, this is backwards.

You do this with all the commands. That is, you create a set of correspondences between IR codes (that you happen to have) hanging around for this) and Apple TV functions. Think of it as totally backwards from what we're used to.

Once you've got this remote programmed... I mean this Apple TV programmed, to respond to this set of codes, you can move on to the next Apple TV and do the whole damn thing again, because you're teaching each Apple TV, one at a time, to respond to those codes. I mean, who would think anyone would ever want to use remote codes for more than one device? Ever? Not Apple.

Thank you, Apple, once more.

EDIT EDIT: Oh, yes, URC says that if you manage to assemble a Frankenstein codeset out of Apple TV 2 and Apple TV 3, some commands will seemingly randomly not work in random apps. The thing that comes closest to working is the reverse learning method.