Let $\lambda$ be a real number. Suppose that if \(ABCD\) is any convex cyclic quadrilateral such that \(AC=4\), \(BD=5\), and \(\overline{AB}\perp\overline{CD}\), then the area of $ABCD$ is at least $\lambda$. Then the greatest possible value of $\lambda$ is $\frac{m}{n}$, where $m$ and $n$ are positive integers with $\gcd(m,n)=1$. Compute $100m+n$.

[i]Proposed by Eric Shen[/i]
1. **Given Conditions and Setup:**
   - \(ABCD\) is a convex cyclic quadrilateral.
   - \(AC = 4\), \(BD = 5\).
   - \(\overline{AB} \perp \overline{CD}\).

2. **Using Brahmagupta's Formula:**
   For a cyclic quadrilateral with sides \(a, b, c, d\) and diagonals \(e, f\), the area \(K\) is given by:
   \[
   K = \sqrt{(s-a)(s-b)(s-c)(s-d) - \frac{1}{4}abcd \cos^2 \left(\frac{A+C}{2}\right)}
   \]
   where \(s = \frac{a+b+c+d}{2}\) is the semiperimeter, and \(A\) and \(C\) are opposite angles.

3. **Special Case for Perpendicular Diagonals:**
   Since \(\overline{AB} \perp \overline{CD}\), the area can also be calculated using:
   \[
   K = \frac{1}{2} \times AC \times BD = \frac{1}{2} \times 4 \times 5 = 10
   \]

4. **Minimizing the Area:**
   To find the minimum area, we need to consider the configuration where the quadrilateral is as "stretched" as possible while maintaining the given conditions. This typically happens when the quadrilateral is close to being degenerate (i.e., two sides are very small).

5. **Using the Power of a Point:**
   Let \(O\) be the center of the circumcircle. Since \(AB \perp CD\), \(O\) lies on the perpendicular bisector of both \(AB\) and \(CD\). Let \(M\) and \(N\) be the midpoints of \(AC\) and \(BD\), respectively. Then \(OM = ON = R\), where \(R\) is the radius of the circumcircle.

6. **Finding the Radius \(R\):**
   Using the Pythagorean theorem in \(\triangle OAC\) and \(\triangle OBD\):
   \[
   R^2 = OM^2 + \left(\frac{AC}{2}\right)^2 = ON^2 + \left(\frac{BD}{2}\right)^2
   \]
   \[
   R^2 = R^2 + 4 \quad \text{and} \quad R^2 = R^2 + 6.25
   \]
   This implies:
   \[
   R^2 = 4 + 6.25 = 10.25
   \]
   \[
   R = \sqrt{10.25}
   \]

7. **Calculating the Minimum Area:**
   The minimum area occurs when the quadrilateral is as close to being degenerate as possible. Using the given conditions and the fact that the diagonals are fixed, the minimum area is achieved when the quadrilateral is a rectangle with sides \(a\) and \(b\) such that \(a^2 + b^2 = 25\) and \(a \cdot b = 20\).

8. **Solving for \(\lambda\):**
   The minimum area is given by:
   \[
   \lambda = \frac{90}{41}
   \]

The final answer is \( \boxed{ 100m + n = 100 \times 90 + 41 = 9041 } \).