Well, this is really not a WD “issue”, for example, if your 4GB of music was on a Seagate drive, it is not a Seagate issue either. It is an iTunes issue: i.e. How to Use iTunes… Nevertheless, you need to know how to do it, and since I just finished doing stuff like this within the past hour it is fresh on my memory.
Open iTunes (I presume you have used it before to perhaps add CDs to your iTunes as many do.)
In upper left corner of iTunes click on File tab and open it. If you want to add a complete album folder, then select Add folder to library, and if you want to add a music track only, click on Add file to library.
You then are asked to select the item you want to add from your computer or My Cloud (if it is on your network). Highlight item, click on the Select button below and item is added to your iTunes database. For the next items, as the saying goes, Wash, Rinse, Repeat.
These are very brief instructions, so if you need more info you can find the full story in the iTunes Help tab!
One thing you do not do, is just copy music files into the iTunes as you would to copy a file to another location. Music must be added (imported) into iTunes via the program as I described.