Can a iphone 4/4s use a iphone 3/3GS use the same screen protectors and vice-versa?

I was wondering because they're about the same size.

Nope. Screen protectors are made by third party manufacturers fit the entire front face of the phone. They get the measurements from Apple via the public release of the dimensions. So although the screen is 3.5 for both, the true front face of a 4 vs. 3gs isn't the same. Cut and trim if you want. Your choice. Best luck.