Table Of Contents Beyond TOC3

I am using Calibre 4.6.0 to convert a Word .docx file to .html for a web-based user manual.

The conversion path is DOCX > HTMLZ > ZIP.

The source .docx file has a table of contents built by Word with four levels extracted from Word's Heading1 - Heading4 styles:
eg:

1 This is a First Level Chapter Title
1.1 This is a Second Level Chapter Title
1.1.1 This is a Third Level Chapter Title
1.1.1.1 This is a Fourth Level Chapter Title

Calibre does a good job of performing the conversion using the TOC from the Word document source and correctly extracts the four levels into the TOC and hyperlinks to the target headings in the text body. A couple of minor .css tweaks corrects for some text misalignment issues at the target headings.

One issue: The fourth level target does not show the chapter level at the target. Example:

1.1.1.1 This is a Fourth Level Chapter Title

shows correctly in the TOC, but without the chapter id of 1.1.1.1 at the target text.

TOC1-TOC3 show the chapter level correctly in the TOC and at the target text.

How to configure Calibre to show the chapter ID for TOC level 4 ?