Looks like iPhone size + Galaxy S class + Android is too hard. My guess is: because to get decent performance from Android you need more MHz/cores/RAM (than iOS), so more battery, so bigger phone.
While a good guess, my guess would be about the money. If there was money in a small-screen Android along those lines, it would exist in the market. They all want to make as much money as they can.
While people here are very passionate about 4" now (and 3.5" back when it was the one and only "perfect" size), I'm doubting the same level of passion for <=4" shows itself beyond the Apple faithful. Someone might lob sales volume numbers for iPhone units to counter but 4" is the ONLY choice if one wants iPhone. If 4" is what the masses want, this 4.7" and 5.5" launch should be a massive failure. However, I think we all know- even those most passionate here- that it won't flop. Personally, I think it will be "best iPhone ever" and draw back a lot of people who went to Android for bigger screens but would rather have iOS (on bigger screens).
And I doubt that 4" screen-size passion will persist as soon as Apple endorses the new "perfect" size(s) coming soon. Personally, I have great expectations that the masses here will readily jump on 4.7" as the ideal "small" screen and 5.5" as the ideal "large" screen and/or ideal middle between phone and tablet. I expect much of this angst about "too big", "need new pants with bigger pockets", "one-handed use" etc will pretty much evaporate within a few days of Apple announcing 4.7" and 5.5", to be replaced by a ton of "which one should I buy?" and "now that I've actually seen one" threads gushing love of the new models. Not much later, I expect "how did we ever get by with such puny screens" if Apple retires the 4" altogether (else, that will wait until they do retire the 4").
I suspect that Samsung, etc are smart enough to recognize that building such a small-screen phone would not sell that well, either by capitulating dominance of that size to Apple or by already having tested it by observing what sold in prior generations of large vs. small Android phones. If they are capitulating 4", they are probably sweating the rumor of Apple taking bites of market share with new sizes at 4.7" and 5.5". If they've shifted focus to bigger screens because smaller ones didn't sell that well to the masses, Apple is just about to benefit from the same kind of test (even with it's own following… including the people that frequent this site).
A few people seem to be clinging to the idea that iPhone 6 will also include a 4" model as loaded as the bigger-screened versions. Personally, I doubt that but I half hope that Apple does roll it out. 3 head-to-head, comparably loaded iPhones that mostly differ by screen sizes would be the ultimate test of this perception of <=4" is "perfection". But, per what we learned in the Samsung trial, even Apple has acknowledged that the market they covet wants bigger-screened phones. I think a good chunk of "us" just missed that very telling reveal from them. So "we" still cling to the official "perfect" of 4" because that is what Apple has for sale right now.