Can I use an iPhone 4 frameware on an iPhone 5?

For example, I have an iPhone 5 with iOS 9 on it (which is terrible) and I want to downgrade to iOS 7 but Apple isn't signing iOS 7 for the iPhone 5, only for iPhone 4 and a few other devices. So can I download an iPhone 4 ispw frameware file on my iPhone 5? I don't think it should be that much of a problem because there shouldn't be too much differences between the devices

Nope.

Yes, there's "much of a problem". Firmwares are device specific.

You can't mix and match firmwares.

And yes, there's a lot of differences between the two devices.

You can download the firmware, but iTunes isn't going to accept it.