How much music and movies can a 4GB DDR 3 laptop hold?

what you are reading probably is the RAM not the HDD (hard disc drive).

RAM is used for running programms,not storing data.

try finding something like 250GB hdd (it might also state the speed,5400 rpm or 7200)

to get an idea, a song can vary from 2-5 Megabytes and a movie usually around 700MB or in an HD movie around 3 GB

also,remember that you can't completely full a drive

and 1 GigaByte=1024 Megabytes