Hi Jon,
I'm assuming you've used a Heading Style to format the level 4 stuff?
I'm on my Office 2007 computer at the moment but, from memory, I think the Word 2010 process is identical:
If you have, to incorporate the Level 4's you need to:
* click somewhere within the existing TOC
* click the References tab
* in the Table of Contents section, click 'Table of Contents'
* on the menu, click 'Insert Table of Contents ...'
* in the Table of Contents dialogue box, towards the bottom, in the box next to 'Show Levels', either click the up arrow until you get to '4', or just type the '4' in