The sensor on the back has enough pixels. It's a question of heat created by the sensor as well as software and possibly hardware requirements.
All depends on how much $$ Samsung put it into the sensor. Honestly, I doubt it's possible due to physical requirements. But, even if it were, who is going to know how to make a camera take that kind of resolution at good quality other than Samsung devs?
If anyone can find the model number of the sensor used in the s4 camera, I can find out if it's possible.
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