The simple interest formula is
[tex]\begin{gathered} A=P(1+rt) \\ P\rightarrow\text{ initial amount} \\ r\rightarrow\text{ interest rate} \\ t\rightarrow\text{ number of years} \\ A\rightarrow\text{ final balance} \end{gathered}[/tex]Therefore, in our case, the final balance is
[tex]A=400(1+5\%*4)=400(1+0.05*4)=400(1+0.2)=400(1.2)=480[/tex]As for the interest,
[tex]I=A-P=480-400=80[/tex]