iphone 3gs button replacement with a 4 button

I was researching at changing the colour of my phone back cover. I then thought that it would look good if the menu button was red.
The problem is i cant find one anywhere, but i can find them for the iphone 4.

What i want to know is will the button of the iphone 4 fit the iphone 3gs? its just the button not the circuitry so its just a matter of will it fit?

Any help would be appreciated.

bought one, tried it, fits