Why does your 4GB ipod only hold 400 songs?

That depends on the sound quality of the songs. Normally the sound of the songs is compressed (made smaller in size) so more songs fit on your iPod. However, a big compression (with a small file size) makes the song lower in quality. Songs you buy are usually of the highest quality, so with the biggest file size. The highest quality (320 kB/s) uses approximately 3 MB per minute, so a 3.5 minute song would use 10 MB. 400 x 10 MB = 4000 MB = 4 GB. If you rip and create MP3s of your own CDs, you are able to select a lower quality that will use less space. For example, if you select 128 kB/s you can fit approximately 1200 songs on the same Ipod.