[DaveBlake]

...the "Recently Added" is only showing the non-4K movies and I can't figure out what I'm doing wrong.

I see quite a bit of confusion in previous posts.

There are fundamentally 2 kinds of custom nodes - "folder" and "filter" - they are distinct and have different syntax and navigation behaviour.

1) "folder" type
These are very simple. They do NOT have <rule>, <content>, <group>, <match>, <limit> or <order> fields.
Only <label>, <icon> and <path> fields define a "folder" type node. They make use of a limited list of (internal to Kodi) virtual file paths which identify the node see https://kodi.wiki/view/Opening_Windows_and_Dialogs.

Example

Code:

<?xml version='1.0' encoding='UTF-8'?>
<node order="1" type="folder">
    <label>Recent Movies</label>
    <icon>DefaultRecentlyAddedMovies.png</icon>
    <path>videodb://recentlyaddedmovies/</path>
</node>

You can get some additional filtering on some folder type nodes by adding parameters to the path, but it is a side effect of how navigation within a node works internally and not intended as a user feature hence there is no official guide to how to use them. But they do have a use particularly if you want to filter artists or albums on the property of their songs.

2) "filter" type
These have the similar syntax to smart playlists and are defined by <label>, <icon> <rule>, <content>, <group>, <match>, <limit> or <order> fields. They do NOT have a <path> field. However some contents can have "path" rules that apply to a phyiscal music or video file location (but not to virtual file system paths)
For example (if all your 2160p movies are in the same folder, filter for that folder)

Code:

<?xml version='1.0' encoding='UTF-8'?>
<node order="9" type="filter">
    <label>Recent Movies 2160p</label>
    <content>movies</content>
    <match>all</match>
        <rule field="path" operator="contains">
        <value>2160p</value>
    </rule>
    <limit>25</limit>
    <order direction="descending">lastplayed</order>
</node>