i have an i4 working in ios4. which version is better for me, and how to upgrade?

Update it from iTunes on a computer as described at the bottom of this article; devices running an iOS version prior to 5.0 can't be updated straight from the device. It will be updated directly to the newest iOS release supported by the device’s hardware, which is iOS 7.1 for devices with at least 512MB of RAM, such as an iPhone 4.

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