4 devices - why does iTunes have to download updates 4 times?
I have 4 iOS devices - why does iTunes have to download each iOS update 4 times? Is there a way to simply save the 2Gb update file?
iPhone 5s, iOS 9.3.4, iPhone 5C plus 2 iPads
By "update file", I assume you are talking about iOS update files.
These differ for the particular type of iOS devices involved. For example, if you had two iPhone 6Ss with the same Apple ID, these could use the same update file.