how to restore my old iPhone 4 from an iPhone 6 backup?

My new iPhone 6 went for a swim and didn't come back out 😟 I recently bought a new MacBook and the only backup I have is from my iPhone 6. I bought my phone from apple and can't afford a new one at this time so I am trying to restore my old iPhone 4 "which my wife has been using" with my iPhone 6 backup but I'm getting this error "The backup “iPhone” cannot be restored to this iPhone because the software on the iPhone is too old." I don't have an iCloud backup.Is there a way to get it my backup ?

thank you