This 4th generation clickwheel model came in 20 and 40GB, the smaller one having a thinner case, because of the thinner Hard Drive. It eventually got developed into the iPod Photo a year later.

Some were sold through HP and they were branded with the HP logo under the iPod mark, I managed to get on by pure luck from ebay a while ago.

iPod (4th Gen/Clickwheel)

iPod (4th Gen/Clickwheel)
Release Year 2004
Capacity 20 GB, 40 GB
Apple Order No. M9282LL/A
Model Number A1059 (EMC 1995) MP103 - HP branded
Actual Capacity 40 GB, 40 GB - HP branded (20 GB actual size)