The rule for rounding is to look at the digit to the right of the place you want to round to. If it's 5 or greater, you round up. If it's 4 or less, you round down.
In this case, we want to round 4.6 to the nearest whole number. The digit to the right of the decimal is 6, which is greater than 5, so we round up.
Therefore, 4.6 rounded to the nearest whole number is 5.