[DaveBlake]

Is there any way to use the folder node type and tell it to only look at non-4K vs. 4K?

No.
BTW it is not that user's "shouldn't" use folder type nodes, happy for users to be as creative with Kodi as they can manage. It is just that it was not created as a user feature and is just a useful side effect in certain situations.

Anyway you can do it with a filter type node:
Recently added 4k episodes (assuming under a "4k" folder)

Code:

<?xml version='1.0' encoding='UTF-8'?>
<node order="10" type="filter">
    <label>4k Episodes</label>
    <content>episodes</content>
    <match>all</match>
    <rule field="path" operator="contains">
        <value>4K</value>
    </rule>
     <limit>25</limit>
       <order direction="descending">dateadded</order>
</node>

You do seem confused @Offsprin about what @LongMan is saying, but I will let him clarify.

Perhaps you should explore smart playlists to see what you can achieve with filter rules, you can create custom nodes with the same rule combination (the definition syntax of xml files is slightly different to xsp, but rules are the same).