Two right triangles share a side as follows: [asy]
pair pA, pB, pC, pD, pE;
pA = (0, 0);
pB = pA + 4 * dir(0);
pC = pA + 5 * dir(90);
pD = pB + 4 * dir(90);
pE = (4 * pA + 5 * pD) / 9;
draw(pA--pB--pC--pA);
draw(pA--pB--pD--pA);
label("$A$", pA, SW);
label("$B$", pB, SE);
label("$C$", pC, NW);
label("$D$", pD, NE);
label("$E$", pE, 2 * N);
label("$4$", pA--pB, S);
label("$5$", pA--pC, W);
label("$4$", pB--pD, E);
draw(rightanglemark(pB,pA,pC,8));
draw(rightanglemark(pD,pB,pA,8));
[/asy] What is the area of $\triangle ABE$?
Since $AB = BD,$ we see that $\triangle ABD$ is an isosceles right triangle, therefore $\angle DAB = 45^\circ.$ That means that $AD$, and consequently $AE,$ bisects $\angle CAB.$

Relating our areas to side lengths and applying the Angle Bisector Theorem, we have that: \begin{align*}
\frac{[\triangle AEC]}{[\triangle ABE]} &= \frac{CE}{EB} = \frac{CA}{AB} \\
\frac{[\triangle AEC]}{[\triangle ABE]} + 1 &= \frac{CA}{AB} + 1 \\
\frac{[\triangle AEC] + [\triangle ABE]}{[\triangle ABE]} &= \frac{CA + AB}{AB} \\
\frac{[\triangle ABC]}{[\triangle ABE]} &= \frac{5 + 4}{4} = \frac{9}{4}.
\end{align*} Now, we see that $[\triangle ABC] = \frac{1}{2} \cdot 4 \cdot 5 = 10,$ so $[\triangle ABE] = \frac{4}{9} \cdot [\triangle ABC] = \frac{4}{9} \cdot 10 = \boxed{\frac{40}{9}}.$