Find all functions $f(x)$ from nonnegative reals to nonnegative reals such that $f(f(x))=x^4$ and $f(x)\leq Cx^2$ for some constant $C$. 
1. **Prove that \( f(0) = 0 \):**
   - Given \( f(x) \leq Cx^2 \) and \( f(x) \geq 0 \) for all \( x \geq 0 \), we have:
     \[
     f(0) \leq C \cdot 0^2 = 0
     \]
   - Since \( f(x) \geq 0 \), it follows that:
     \[
     f(0) = 0
     \]

2. **Prove that \( C \geq 1 \):**
   - Assume for contradiction that \( C < 1 \). Then for positive \( x \):
     \[
     f(x) < x^2
     \]
   - This implies:
     \[
     f(f(x)) < (f(x))^2 < x^4
     \]
   - However, this contradicts the given condition \( f(f(x)) = x^4 \). Therefore, \( C \geq 1 \).

3. **Define a function \( i(x) \):**
   - Let \( f(x) = i(x) \cdot x^2 \) for some function \( i(x) \). Given \( f(x) \leq Cx^2 \), we have:
     \[
     i(x) \leq C
     \]
   - Since \( f(x) \geq 0 \), \( i(x) \geq 0 \). Also, \( i(x) > 0 \) because if \( i(x) = 0 \), then \( f(x) = 0 \) and \( f(f(x)) = f(0) = 0 \neq x^4 \).

4. **Use the given equation \( f(f(x)) = x^4 \):**
   - Substitute \( f(x) = i(x) \cdot x^2 \) into the equation:
     \[
     f(f(x)) = f(i(x) \cdot x^2) = i(i(x) \cdot x^2) \cdot (i(x) \cdot x^2)^2 = x^4
     \]
   - Simplify:
     \[
     i(i(x) \cdot x^2) \cdot (i(x))^2 \cdot x^4 = x^4
     \]
   - Since \( x \neq 0 \) and \( i(x) \neq 0 \), divide both sides by \( x^4 \) and \( (i(x))^2 \):
     \[
     i(i(x) \cdot x^2) = \frac{1}{(i(x))^2}
     \]

5. **Bound \( i(x) \):**
   - Given \( i(x) \leq C \) and \( i(x) \cdot x^2 > 0 \):
     \[
     i(i(x) \cdot x^2) = \frac{1}{(i(x))^2} \leq C
     \]
   - This implies:
     \[
     (i(x))^2 \geq \frac{1}{C} \quad \text{and} \quad i(x) \geq \frac{1}{\sqrt{C}}
     \]
   - Also:
     \[
     \frac{1}{(i(x))^2} \geq \frac{1}{\sqrt{C}} \quad \text{and} \quad (i(x))^2 \leq \sqrt{C} \quad \text{and} \quad i(x) \leq C^{\frac{1}{4}}
     \]
   - Therefore, \( i(x) \) is in the range:
     \[
     \left[\frac{1}{\sqrt{C}}, C^{\frac{1}{4}}\right]
     \]

6. **Iterate the bounding process:**
   - Define the range of \( i(x) \) after \( n \) iterations:
     \[
     \left[C^{-\frac{1}{2 \cdot 4^n}}, C^{\frac{1}{4 \cdot 4^n}}\right]
     \]
   - As \( n \to \infty \):
     \[
     \lim_{n \to \infty} C^{\frac{1}{4 \cdot 4^n}} = 1 \quad \text{and} \quad \lim_{n \to \infty} C^{-\frac{1}{2 \cdot 4^n}} = 1
     \]
   - Therefore, \( i(x) = 1 \) for all positive \( x \).

7. **Conclude the function \( f(x) \):**
   - Since \( i(x) = 1 \), we have:
     \[
     f(x) = i(x) \cdot x^2 = x^2
     \]
   - This function satisfies \( f(0) = 0 \) and \( f(f(x)) = x^4 \).

The final answer is \( \boxed{f(x) = x^2} \).