I have been replacing iphone screens for a couple years now. 3G 3GS 4 and 4S.
I have had to substitute a 4 for a 4S and vice versa a couple times. Of course, it is best to use the proper model whenever possible, but here's a trick if you absolutely have to use the wrong screen [and therefore the wrong bezel].
Chopping off the brackets can be very hard to do unless you have the right sort of metal cutter / industrial scissors.
Instead, you can carefully remove the bezel from both the cracked (original) screen and the new screen and reattach the original bezel to the new screen.
A couple things to be careful of:
If you pull too hard on the bezel, you will snap it / crack it. Not the end of the world (you can still use it), but be slow and deliberate when pulling away from the screen/LCD.
Also, if you pull too hard and with too much force, you might crack the LCD and have the dreaded bleeding ink drip look negating your whole endeavor.
Carefully start by pulling on a corner with your thumb pushing the screen away and your fingers pulling toward you on a corner bracket.
Make sure you pull the sticky tape along with the bezel and keep it in place as you go (it will try to pull away). This is extremely important as that sticky tape is the ONLY thing holding the screen to the rest of the body.
This is a drastic step, but it does work.
I know I have replaced 4 screens with 4S without a problem. I can't say with confidence that it works the other way around.
If you're getting white lines running through your LCD, you've probably bought a cheap part.
It's best to buy a part (ebay, for example) that is held by a warehouse distributor in the USA. This ensures that you can track the part, hold someone accountable and usually expect a more timely delivery.
If you order the part from Hong Kong, you're ordering from a black hole. Not to say that it won't work, but you're taking a little more of a chance.
Also, DON'T BE CHEAP! Spend the extra $10-20 bucks and get yourself a nice screen. Don't just buy the one that's the best deal.
Andrew