If you download music or video files a lot then your iPhone can quickly run out of space. You could find out how much space you have got left on your iPhone 4 so that you don't overfill your device hard disk space. Here's how:
1. Go to the Settings screens.
2. Next tap on General.
3. Then tap on About.
4. From the About screen navigate down to the section Space available and you can find out how much memory you've got left. In this About screen you will see other information about your device too.
See iPhone 4 User Guide Page 189 to learn more about your device