When you are deciding what to put on your 4 GB iPhone, there's more to consider than how many songs, or how many movies, per gigabyte -- because file sizes and types vary. More important than that is the fact everyone mixes and matches their media differently. One person might use his iPhone for apps and photos, while another may be using it for all of her media on top of apps: songs, movies and pictures.
The entire 4 GB on your iPhone is not free space; some of that space is taken up by the operating system and relevant files, between 400 and 800 megabytes depending on your version of iOS. Bear in mind that video files are significantly larger than music files; the more video you have, the lower your capacity for songs will be. When connected to iTunes, the program will not only display how much free space you have left, but also how your space is divided.
The official stance on 4 GB iPod/iPhones is that you should be able to fit 1,000 songs on your device. However, this assumes two things: one, that there's no other media on the device, and two, that songs are encoded at 128 kbps and last for four minutes. If you have lower-quality audio, or shorter songs, you may find that you can fit slightly more; likewise, higher-quality and longer tracks will affect how many songs can fit.
The average full-length movie from iTunes is about 1 GB; this makes the 4 GB iPhone a bit impractical for taking movies on the go. However, if you're converting your own videos to iPhone/iPod format, you can lower the quality and resolution and end up with smaller files. Thirty minute TV episodes tend to be under 200 MB, which is more practical for a 4 GB device.
How many apps you can fit on your iPhone varies, depending on the app itself -- most apps measure in megabytes, with games being the most resource-heavy. The 2010 release of iOS 4 introduced the ability to organize your apps into folders. An iPhone running iOS 4 can store up to 180 folders, with each folder holding up to 12 apps. If you don't use the iPhone for music or movies, you should have little issue with apps and space.
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