Wolf-uk , first step is to format your player. As there is no 4GB nano-clones in the market at the moment. These players only supports a max of 2GB.
Next, backup your firmware. The steps are
Here.
Once you have the repaired firmware, keep it safely.
Now, to theme the player,
Here is a guide on how to modify the firmware. Note, ALWAYS use your own backup firmware! Never use someone else's firmware, it would very likely kill your player.
Here is another guide on theming your firmware. I'll suggest you have a good read on these guide before starting on the theming.