Just like gas tank size isnt a measurement for how long a car can drive on a full tank, number of GB isnt a measure for how many minutes of video or music you can put on there. You can however do estimates:
The maximum video bitrate of the sony is 768, with audio bitrate for video of max 288 or so. Lets call that max total bitrate of 1mbps. 4GB = roughly 32000 mbit, which means you can at LEAST have 32000 seconds = 533 minutes aka ~9 hours of video on it, if you dont have any music on there and use absolute max video quality.