iOS 5 will be compatible with iPhone 3GS as well as iPhone 4.
Sadly, no. The latest version of the 3rd iPhone (iPhone 3GS) is iOS 6.1.5.
The current latest iOS version on the iPhone 4S is iOS 6.1.3 considering iOS 6.1.4 is for the iPhone 5 only. iOS 7 will be available for the iPhone 4S this fall.
Yes, you can run iOS 7 on the iPhone 4.
At the moment iphone 6 runs ios 8.1.3
The iPhone 5 will run iOS 6 or iOS 7.
The iPhone 4 can only support from iOS 4.0 to iOS 7.1.2. There is no way to get any higher iOS Firmware due to the older hardware that is built into the iPhone 4.
iOS stands for iPhone OS. When the iPhone Was first released, iOS was called iPhone OS 1.x.x. Today, on iOS 8, made people so mad bc iOS is a locked system meaning "un customizable "
The iPhone 4 can currently run up to the latest version: iOS 7.1.
Since ios 7 is designed for the iphone 5, it will probably make the iphone 4 slower.
This was created to add support for the Verizon iPhone. No device uses it except the Verizon iPhone 4, as the same feature was added to iOS 5, and the iPhone 4S included iOS 5. It is essentially iOS 4.2.3 for Verizon.
Yes you can, but expect some iOS functionalities that are on, say the iPhone 5S won't carry over to the iPhone 4.