saher_arts,
Your player is most likely only 1GB. I have yet to see any proof that 4GB players really exist. At the moment the largest capacity player available is 2GB.
The players are hacked so that they read as being larger than they really are so a 512MB or 1GB chip can be hacked to appear as a 4GB or 8GB chip. Ususally the players have problems with file corruption and player restarts when you try to access any of the files that are above the true chip size. When the player is formatted it removes the hack and the true size of the chip is revealed.
To be certain you would need to open the player and note the serial number and make of the memory chip (the largest chip on the board), doing a Google search with this information will find you the chips real specifications.
You should really contact your seller and demand a full refund. If you look through the forums you will find posts by others with the same problem.
http://www.mympxplayer.org/viewtopic.php?t=1323
http://www.mympxplayer.org/viewtopic.php?t=1290
http://www.mympxplayer.org/viewtopic.php?t=424
http://www.mympxplayer.org/viewtopic.php?t=1157