On the back of the packaging you can see highlights of all the important specs. For the most part the devices include identical features, including Wi-Fi 802.11n support, GPS, Bluetooth, DivX/Xvid video support, a 3.2-megapixel camera, and a 1GHz processor.
The Galaxy Player 5 has the unique bragging rights of a larger 2,500mAh battery, presumably for improved battery life. The Player 4 offers a "Super Clear LCD" not available on the Player 5.
The front of the Galaxy Player 4 features a 4-inch screen with an 800x480-pixel resolution. Samsung calls the screen technology Super Clear, which is perhaps a reference to the panel's pixel-density, similar to Apple's Retina Display technology.
As expected, the 5-inch screen on the Galaxy Player 5 allows plenty of screen real estate. Unfortunately, the pixel density clearly isn't as sharp as on the Galaxy Player 4, and the image quality pales in comparison to Samsung's AMOLED screen, used on the Galaxy S II smartphone. Screen resolution is the same 800x480 pixels used on the Player 4.
From left, we have the backs of the Galaxy Player 5, Galaxy Player 4, and Apple iPod Touch. The iPod is by far the thinnest and most pocketable, though its screen size is relatively small at 3.5 inches.
From left, we have the Galaxy Player 4, Galaxy S II smartphone, and Galaxy Player 5. Even with the screens turned off, it's clear that the S II smartphone in the middle is using a different technology (AMOLED) with deeper blacks.
With the screens turned on, you can see that all three Samsung Galaxy devices use a very similar home screen and interface. Aside from the differences in screen quality, one key difference is the choice of apps used in the dock (which Samsung prevents you from customizing).