OP think about it this way
Its not the phone that is scratching easier, its that the screen resolution is higher and you notice the scratches more
a) Higher resolution means you hold the phone closer now
I honestly don't think the higher resolution would make any difference to how easily scratches are noticed. And why would a higher resolution make you hold your phone closer? I suppose it means it looks sharp up to a closer distance, but it's not good for your eyes to bring it closer to your face.
The thread starter said that they noticed one scratch whilst reading under sunlight. Bright light, such as sunlight, shows up all the blemishes on the screen very easily. I'd like to know, however, whether or not it's a deep scratch we're talking about.
OP, can you feel the scratch with your nail? If not, it's just a surface scratch in the oleophobic coating, not the glass screen. It's very easy to get scratches in the coating (it's a ludicrously thin layer of oil-repellent plastic) and these are considered normal. You should only be really concerned if the scratch is deep into the glass and catches a fingernail as you run it over.
I had a deep glass scratch on my iPad (which thankfully got replaced due to dust under the glass). I had no idea how it happened, since I had been babying it and keeping it inside a case (that covers the front when not in use). I'm just saying that glass scratches do happen on all iOS devices by chance.
The iPhone 4 is no less susceptible - in fact, it has the toughest glass so far in an iOS device. You can still get unlucky and suffer scratches in it, though. It's not scratch-proof, only resistant.