Re: Why has a black shadow appeared at the top of my screen
That sounds like one of two problems - water damage (which can also occur from condensation if the phone gets very cold for hours, then is brought right into a warm house) or a broken screen. (The LCD part of the screen can be broken without the glass breaking.)
In either case it'll have to be repaired. If it's water damag, and not too extensive, it would still pay to repair. (If the entire phone is full of mold and shorts, it's a lost cause, because a replacement phone is cheaper). If it's just a damaged screen, an S4 is still worth repairing.
If you decide to replace it, Swappa is a reliable place to find good used phones. (Don't buy a phone from eBay or, worse yet, Craig's List.)