If you can only fit 140 songs on a 4GB iPod mini and are dissatisfied, you most likely picked the wrong compression method for your songs. Apple assumes that you will save your music files as 128kbps AAC or MP3 files, but iTunes gives you a variety of larger filesize options that can dramatically reduce the number of songs an iPod can hold. You may have seen “Apple Lossless” as a choice; this is most likely to create the 140-song problem. Our simple advice would be to set iTunes’ preferences for Encoding to encode files as 192Kbps MP3, or 160Kbps AAC, select all of your songs at the same time, and right click (PC) or control-click (Mac) to select “Convert Selection to…”? MP3 or AAC. The new files you create will most likely fit on your iPod and leave room to spare.
Alternately, it’s possible that some (but not all) of your music is uncompressed, or that you use your iPod as a hard disk as well. If you have other files on your iPod, there is less room available for music. You can read more about using the iPod as a hard disk in this article.