[tadad1]

neomuzic,
As "bsaylor" pointed out anything above 2GB is a myth as far as I am concerned as I have seen no conclusive proof that they really exist.
If the seller has warned you not to format the player indicates to me that he is aware of the hack so I would not expect to much help there.
Most of these players run on a ATJ2085 chip which as "altean" pointed out in a post will only support a maximum 2GB flash memory according to the chips spec sheet.
You need to open your player and find the memory chip which will be the largest one on the board and note the serial number and brand and doing a google search with the info will find you your chips true specs.

Sorry there is no way to get it to a 4GB chip again as it was never that big to begin with.