Understand the positions of decimal places. Decimals are typically extended two places for fourth graders. The first space after the decimal sign is tenths. The second space is hundredths. So, you would read the decimal 4.2 as four and two-tenths.
Understand where the decimal lies on a number line. Decimals are in-between whole numbers. First, move down the number line to the whole number used and then work slightly past the whole number for the decimal. So, 4.2 would be just past four on the number line.
Convert decimals to fractions by dividing by 10 or 100, then simplifying if necessary. So, 4.2 is stated as four and two-tenths. This becomes 4 2/10, which is simplified to 4 1/5.