Set up the long division.

We need to divide 4 by 5. Since 4 is less than 5, we will get a decimal answer.

$$\begin{array}{r} 0. \\ 5 \overline{\smash{)} 4.0} \\ \end{array}$$

5 does not go into 4. So, we place a 0 in the quotient above the 4 and add a decimal point. Then, we add a 0 to the 4 to make it 40, effectively dividing 40 tenths by 5.

$$\begin{array}{r} 0.8 \\ 5 \overline{\smash{)} 4.0} \\ \end{array}$$

Divide 40 by 5.

$$40 \div 5 = 8$$

Place the 8 in the quotient above the 0.

$$\begin{array}{r} 0.8 \\ 5 \overline{\smash{)} 4.0} \\ -40 \\ \hline \\ \end{array}$$

Multiply 8 by 5:

$$8 \times 5 = 40$$

Subtract 40 from 40:

$$40 - 40 = 0$$ $$\begin{array}{r} 0.8 \\ 5 \overline{\smash{)} 4.0} \\ -40 \\ \hline 0 \\ \end{array}$$

The remainder is 0, so the division is complete.

The final answer is $\text{0.8}$.