According to the Galaxy Player Wikipedia page the differences are:
Galaxy Player 4 (YP-G1) The Galaxy Player 4 features a 4" Super Fine LCD.
It had a flash internal storage of 8
GB or 16 GB that can be expanded with
microSD card.
Galaxy Player 5 (YP-G70) The Galaxy Player 5 features a 5" TFT LCD.
It had a flash internal storage of 8
GB or 16 GB that can be expanded with
microSD card.
Galaxy Player 50 (YP-G50) The YP-G50 model carries a 3.2" LCD TFT
instead of the AMOLED which is used by
Samsung in most of its phones. The
screen has a 240×400 resolution. It
has a flash internal storage of 16 GB
or 32 GB that can be expanded with a
microSD card.
So the main differences are the size and quality of the screen (and the device), and the internal memory capacity.
Essentially the "50" has the smallest, worst quality screen, but (depending which capacity you get) has the most internal memory, the "4" and "5" are larger devices with better quality, larger screens but (potentially) not as much internal memory.
According to another review I found:
...4.0 and 5.0 have almost the same specifications... the 4 inch version has a battery
capacity of 1200mAh, whereas the 5
inch version is 2500mAh
So the "5" has a larger battery which should last longer, though this may be offset slightly by the larger screen, and will also make the device slightly bigger and heavier than the "4". Battery life should be better than equivalent phone models (especially if you switch off wifi when you're not using it), as the phone's wireless radio is one of the largest battery hogs in the device. Full size and weight specs of the devices are in the sidebar on the Wikipedia page.
So it looks like none of the devices are "better" than the others, they all appear to have their own benefits but are aimed at different buyers.