How come arccos(cos2) = 2, but arccos(cos4) = 2.28

I suppose it has to do with the range of arccos, but where exactly does 2.28 come from?

Thanks.

Because 2 radians is within the first " loop" of the cos curve, but 4 radians is beyond a revolution, so it defaults back to 4 - 2π and this gives the 2.28

because it does