Did you try the second link I mentioned?
I also found this comment via a Google search which might help:
"On the first generation nano, the front of the player was a glossy plastic and the back of the player was a glossy metallic finish. Both sides were extremely prone to smudges and fingerprints. With the second generation nano, the materials have changed on the front and back (now anodized aluminum), and it doesn't smudge or leave behind fingerprints, which makes it look a lot nicer once it's actually in use."
Also this link
http://the-gadgeteer.com/2007/01/22/apple_ipod_nano_2g/ has a picture showing a 1G and a 2G Nano. The 1G Nano appears to have rounded corners, when viewed from above. The same article mentions the 2nd Gen Black is 8GB, which implies your 4GB Black is a first gen.