$a$ and $b$

$\displaystyle \begin{aligned} 400 - \overline{ab4} & = \overline{4ab} - 400 \\ 400 - (100a + 10b + 4) & = (400 + 10a + b) - 400 \\ 396 - 10(10a + b) & = 10a + b \\ 396 & = 11(10a + b) \\ 36 & = 10a + b & \small \color{#3D99F6} \text{Note that} \ 10a + b = \overline{ab} \\ \boxed{36} & = \overline{ab} \end{aligned}$

$\begin{aligned} 400 - \overline{ab4} & = \overline{4ab} - 400 \\ 400 - 100a - 10b - 4 & = 400 + 10a + b - 400 \\ 396 & = 110a + 11b \\ \implies 11\overline{ab} & = 396 \\ \implies \overline{ab} & = \frac {396}{11} = \boxed{36} \end{aligned}$

$400-\overline{ab4}=\overline{4ab}-400$ $400-(100a+10b+4)=400+10a+b-400$ $400-100a-10b-4=400+10a+b-400$ $396-100a-10b=10a+b$ $396=110a+11b$ $396=11(10a+b)$ $\dfrac{396}{11}=36=10a+b=\overline{ab}$