Rechargables are usually 1.2v - 1.25v, 4 x 1.2 = 4.8 which is certainly good enough
Remember you'll need NiMH batteries that are over 1300mah to actually charge the player, I did a test with 4x 1300mah AA's and it didn't properly charge the internal battery, 4x 1800mah's charged it fine.