You didn't ask but I'll give you my perspective...
Basically someone who loves the size of the 4" screen feels that (A) one-handed use and easier pocket-ability outweighs (B) a bigger screen. As such a person, this is my reasoning:
- I really want something that I use as often as my phone to be used comfortably and securely with one hand; not having to stop walking to use it, not having to put things down to use it, not having to balance it in one hand, not worrying about it falling if I take a wrong step or someone bumps my elbow. One-handed usability is always better when we can have it, and I think that in this non-ideal, unpredictable, on-the-go world, it's very important on a phone.
- I really don't like having anything in my pocket. I do because I have to, but the bigger the worse.
- Bigger screens are better when you're doing things like web surfing, reading, watching videos, or looking at photos; that's indisputable. I could see if you do those things often enough on your phone (because an iPad is not handy or if you simply don't use iPads), then a bigger screen would be valuable. But here's where I differ (I don't know what the case is for other 4" phone users): I don't often do those things outside of the house; for me they're almost exclusively done inside the house (and either on my iPad, computer, or TV), and very intentionally so. It's mainly because of two preferences I have in life: I like to have as much face to face human interaction as possible, and I like to be as aware of my environment as I can be. These "philosophies" apply mainly outside the home because home is a known, private, and controlled environment. Smartphones are extremely useful, but they're also riddled with distractions. So when I'm out of my house, I try to use my phone as minimally as possible. Quick lookups, quick emails, etc., very rarely spending any substanitial amount of time looking at my screen (unless perhaps it's work-related). Then when I get home on my comfy couch, I read/watch/type away on whichever device is best suited for the job, which is usually not my phone. This makes a big iPhone screen simply not very important to me.