I'm afraid that neither Splunk's Simple xml format nor the underlying advanced XML format support XInclude, nor any other way for you to include parts of the XML from outside the file itself.
to give some hope, I am working on a way to essentially get this feature as a part of a future version of Sideview Utils but it's not ready yet and most likely won't appear until 3.0.
However, if you have 4K lines of XML, and you're not already using Sideview Utils, you should check it out - Assuming this is an Advanced XML view and you're using only core Splunk modules still, switching to the modules in Sideview Utils can cut that number of lines roughly in half, and remove a lot of the indentation complexity. Here's a 10min video on youtube that's more or less on this topic:
... View more