Some people may have been surprised to find that they were told to delete some memory on their Apple device before being able to install iOS 8. The update itself requires 5.7GB of free space to be able to successfully install.
This may seem like a sizeable chunk, but it is only needed during the install. The large file size is down to the system having to temporarily store multiple copies of the download. Firstly it’s the compressed file, which then needs to be decompressed and installed alongside the already operating iOS 7. When the update is completed, the compressed file and iOS 7 will be deleted, freeing up more space.
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However, you can avoid having to delete personal files forever in order to free memory. There are 2 main options: option one is to back up your files, delete the large ones from the device and re-add them later or, option 2, you can just perform the update through iTunes which eliminates the need to delete anything altogether.
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iOS 8 is available for the iPhone 4s and later and includes a new HealthKit software suite, QuickType prediction, Continuity features for syncing with a MacBook, and improvements to widgets and Siri. Although it is available on iPhone 4s, it isn’t necessarily recommended as you won’t get all the new features, and it may cause the phone to run a lot slower due to it simply not having the hardware power to support it. But each to their own.
Source: IGN