document.xml <w:pStyle w:val="Heading4" vs <w:pStyle w:val="4"/>

See also: STYLEREF Fields and Language-specific Style Names by Lene Fredborg, MVP

I have uploaded the testing file to here, see if this helps, thanks.

https://easyupload.io/mc6vvr

I cannot create that XML using Word 2016 MSO (Version 2406 Build 16.0.17726.20078) 32-bit. Applying built-in heading styles in that version creates the HeadingX syntax. This is also what happens with all other versions of Word I've tested.

Editing the XML to the X syntax causes all styles text to revert to Normal style. This is also what happens with all other versions of Word I've tested.

It appears your copy of Word is very out of date, it should be updated.

as follow:

the top part of document.xml, if useful for investigation, is as follow

http://schemas.microsoft.com/office/word/2010/wordprocessingCanvas" xmlns:cx="http://schemas.microsoft.com/office/drawing/2014/chartex" xmlns:cx1="http://schemas.microsoft.com/office/drawing/2015/9/8/chartex" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships" xmlns:m="http://schemas.openxmlformats.org/officeDocument/2006/math" xmlns:v="urn:schemas-microsoft-com:vml" xmlns:wp14="http://schemas.microsoft.com/office/word/2010/wordprocessingDrawing" xmlns:wp="http://schemas.openxmlformats.org/drawingml/2006/wordprocessingDrawing" xmlns:w10="urn:schemas-microsoft-com:office:word" xmlns:w="http://schemas.openxmlformats.org/wordprocessingml/2006/main" xmlns:w14="http://schemas.microsoft.com/office/word/2010/wordml" xmlns:w15="http://schemas.microsoft.com/office/word/2012/wordml" xmlns:w16se="http://schemas.microsoft.com/office/word/2015/wordml/symex" xmlns:wpg="http://schemas.microsoft.com/office/word/2010/wordprocessingGroup" xmlns:wpi="http://schemas.microsoft.com/office/word/2010/wordprocessingInk" xmlns:wne="http://schemas.microsoft.com/office/word/2006/wordml" xmlns:wps="http://schemas.microsoft.com/office/word/2010/wordprocessingShape" mc:Ignorable="w14 w15 w16se wp14">

Not just 4, but also 1,2,3

<w:pPr><w:pStyle w:val="1"/></w:pPr>

<w:pPr><w:pStyle w:val="2"/></w:pPr>

<w:pPr><w:pStyle w:val="3"/></w:pPr>

<w:pPr><w:pStyle w:val="4"/></w:pPr>

Which versions are you testing?

In Word 2007 and the current version of Word 365, neither one successfully interprets

<w:pPr><w:pStyle w:val="4"/></w:pPr>

to apply the built-in Heading 4 style.