燒碟的CD上面 有些 4X, 48X 是什麼意思??

the 4x, 48x mean the certified highest recording speed for that particular media,
CD-R = CD - Recordable
CD-RW = CD - ReWritable
in CD-R data cannot be erased and they will stay permently on the disc, but you CAN burn the disc again if you didn't close the track in the previous buring session.
in CD-RW , data can be erased completely and burnt again like a new black disc.
cheers