Can you download the ios 7 for iPhone 4 and put it on iPhone 3GS?

What would happen if i put the ios 7 for iPhone 4 on a 3GS?

It wont accept it. When you try to put it in it verifies with Apple and it will give you an error. There is no way to do it.

There will be ioS 7 for all I devise. But for the 3GS it will be slower than other I devise as the 3GS as they thought it wouldn't last that long and people would stop using them. But will work!

You can't. That is not how iOS works. Can't trick it into thinking it is running on something. All iOS devices give off electronic signatures which can not be changed.

No way of getting iOS 7 on a 3GS or lower.

It can because I have it