That's not a bug. It looks like that in real life with mist or fog at a distance on the ocean and has to do with the way the sun shines though it. It wasn't in SH3 because the rendering engine wasn't as good. So in reality, SH3 is the one with the bug.
This has been discussed before here and quite a number of pictures of real sunrises and sunsets have been posted to prove this. How dark the area on each side is depends on how much mist / fog there is. Light fog / low humidity and its barely noticable. Thicker fog / higher humidity and it becaomes very pronounced.
Might get a better reception if you ask if something is a bug before yelling about it (Yes large bold print on a forum is yelling). People here have only been playing SH4 since it first came out. Give them some credit.
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"There are only two types of ships- submarines...... and targets" Unknown
"you wouldn't catch me on a ship that deliberately sinks itself"- comment to me from a surface sailor. 
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