Thank you for your inquiry! I'd be happy to assist.
First, there is no way to directly connect an external hard drive directly to your Wave SoundTouch Music System IV via USB. To play this music, you will need to utilize SoundTouch's Music Library source. The hard drive will need to be connected to a computer.
Here are instructions: Adding a music library to SoundTouch®
If you don't wish to use a computer, you could consider transferring this music to a Network Attached Storage (NAS) device. These are devices that are always on, and act as a data-storage device, as well as a home-server, used to access files and media from other devices on your home network. For more information, see this article.
I hope this helps! Please let us know if you have any additional questions.