When you are rounding numbers, if the integer to the right is <5, then you keep your rounding number. If it is >5, then you add 1 place value to your rounding number.
Therefore, if you are trying to round 4.125 to the nearest whole number, you are trying to round it to the ones place, or where the 4 is.
Because 1<5, 4.125 rounded to the nearest whole number is 4.