The zoom lever on this keyboard is supposed to act something like control-mouse-wheel does in a Web browser or Microsoft Office products. You can flick the lever one way or the other and the text or picture zooms in or out.
The functionality was implemented by Microsoft's IntelliType driver. The final driver release was August 2011. Nevertheless, Microsoft still provides downloads for drivers which could work for older products such as the Natural Ergonomic Keyboard 4000. Microsoft has an Accessories Support site with a page “Which devices are supported by Microsoft Mouse and Keyboard Center?” It discusses older products which rely on IntelliType. The advice there recommends not installing the latest Microsoft Mouse and Keyboard Center software release. Instead, it recommends either:
- installing version 2.3 of the same software
- installing the plain IntelliType driver
Download links are provided for each recommendation. For what it's worth, I followed the recommendation on a Windows 10 system with a different vintage 2011 keyboard—Microsoft Digital Media Pro—and its zoom feature now works as it did originally.