I think I see what you're saying. Yes, I agree, there are some people who can maintain an unreal level of focus for 4 hours - but most people aren't like that. (I'm thinking in particular of the pro tennis players competing at Wimbledon right now. There are always some players who are very talented physically but can't maintain the level of intensity consistently; the ones who end up being the great champions are the ones who can maintain that intensity.)
I'm going to call out a particular sentence in your post:
There are some questions which I just cant answer in the given time owing to their difficulty level; however, going by common perception, the simpler ones should all be done correctly if only one can focus hard.
For the first half of your sentence, you know that you should give up on those at or before the given time limit, yes? Make an educated guess and move on - don't waste time on those. (And you'll
always have some questions like this, no matter how good you get.) For the second half of your sentence, I mostly agree, but I would replace the word "all" with the word "most." No matter how well we can focus, we still make some careless mistakes - that's just life. If you try to hold yourself to the "I must never make a careless mistake" standard, well... you're not going to be very happy. :) Instead, we should think about trying to minimize careless mistakes as much as is humanly possible - but that's all.
In order to minimize careless mistakes, we actually need to understand both what mistakes we're making and WHY we're making them. So keep an error log and write down precisely
why you get any problem wrong. Then figure out how you can change your methods or institute new habits in such a way that you will minimize the chances of repeating that type of error in the future. (Could be anything from: "I messed up the formula, so I will drill the formula until I don't mess it up" to "I made a simple math error because I didn't fully write out my work, so I will write out all of my work from now on" to "I overlooked an important qualifying* word in the sentence and therefore misunderstood the meaning, so I will write down and underline qualifying words in the future.")
(* A qualifying word is something like "
all of the employees", vs just some, or "he
never studied" vs. he did study, but rarely.)
If you spend some time studying why and how you tend to make careless mistakes, you can institute new methods / habits that will reduce your chances of making those same mistakes in the future. That may help with the focus problem you describe, because you're reducing opportunities to make mistakes in the first place.