Why is 4...Nc6 inaccurate?

I cannot seem to see why the evaluation suddenly went up for White by 0.7 after my 4...Nc6.

Maybe the computer doesn't like humans. Maybe it thinks the bishop could become bad. Maybe an input error was made. Maybe the database shows that white has a 60% win rate from this position. Maybe, as many others have said, the computer can't evaluate with so many pieces.

pretty sure modern consensus is that white is better after 5 f4 :(

Maybe after Nc6, the computer suggests that a3 is best. However, after a3, it gives a + 0.1. It is just a bug. Or, e6 would have put pressure on the pawn maybe might be why also? Inaccuracies mean that the move is ok, so you won't have to worry about it too much.

maybe computers does not think as same as humans

Don't worry 4.Nc6 is main line. Such fluctuations in the opening don't mean anything. Unless the evaluation goes from -ve to +ve with a deviation greater then 0.5 it isn't an opening mistake. Eg if eval goes from -0.1 to 0.1 then it isn't a mistake.
Anyways, an advantage of less then 1 doesn't mean much, even more so in the opening.

5. f4 e6 6.Ae3 d4 7.Af2 Axc5 8.Cd2 Cge7 9.a3 Cd5 10.Df3 O-O 11.Dg3 b5 12.Cf3 b4 13.Ad3 Ce3 space advantage.

computers are not very good at opening theory analytics because they are used for middlegame analysis. Using a chess book would be the best way to study this opening. don't listen to the computer

There is a consensus of many good players that the … c5 CK advance is practically only equalizing with 4. … e6. The books like the Informant point at that. I switched from Nc6 to e6 myself, Nc6 ends with advantage White. It used to be main line but times have changed.

Actually I am not sure whether the machine recognizes that or it is a horizon issue. Anyway, it seems to be right.

On amateur level though it might be a minor issue, true.