Determine all solutions of the equation $4^x + 4^y + 4^z = u^2$ for integers $x,y,z$ and $u$.
To determine all solutions of the equation \(4^x + 4^y + 4^z = u^2\) for integers \(x, y, z\), and \(u\), we can proceed as follows:

1. **Rewrite the equation in terms of powers of 2:**
   \[
   4^x + 4^y + 4^z = u^2 \implies 2^{2x} + 2^{2y} + 2^{2z} = u^2
   \]
   Without loss of generality, assume \(x \geq y \geq z\).

2. **Case 1: \(x = y = z\):**
   \[
   2^{2x} + 2^{2x} + 2^{2x} = u^2 \implies 3 \cdot 2^{2x} = u^2
   \]
   For \(u^2\) to be a multiple of 3, \(u\) must be a multiple of \(\sqrt{3}\), which is not an integer. Therefore, there are no solutions in this case.

3. **Case 2: \(x = y \neq z\):**
   \[
   2^{2x} + 2^{2x} + 2^{2z} = u^2 \implies 2 \cdot 2^{2x} + 2^{2z} = u^2 \implies 2^{2x+1} + 2^{2z} = u^2
   \]
   Let \(2z = k\), then:
   \[
   2^{2x+1} + 2^k = u^2
   \]
   If \(k = 2x + 1\):
   \[
   2^{2x+1} + 2^{2x+1} = u^2 \implies 2 \cdot 2^{2x+1} = u^2 \implies 2^{2x+2} = u^2 \implies u = 2^{x+1}
   \]
   This gives a solution \(x = y\) and \(z = x + 1\).

4. **Case 3: \(x \neq y \neq z\):**
   Assume \(x > y > z\). We need to check if there are any solutions in this case:
   \[
   2^{2x} + 2^{2y} + 2^{2z} = u^2
   \]
   If \(2y = x + 1\):
   \[
   2^{2x} + 2^{x+1} + 2^{2z} = u^2
   \]
   Let \(2z = x + 1\):
   \[
   2^{2x} + 2^{x+1} + 2^{x+1} = u^2 \implies 2^{2x} + 2 \cdot 2^{x+1} = u^2 \implies 2^{2x} + 2^{x+2} = u^2
   \]
   This simplifies to:
   \[
   2^{2x} + 2^{x+2} = u^2 \implies 2^x (2^x + 4) = u^2
   \]
   For \(u\) to be an integer, \(2^x + 4\) must be a perfect square. Let \(2^x + 4 = k^2\):
   \[
   2^x = k^2 - 4
   \]
   Solving for \(x\) and \(k\) gives:
   \[
   k^2 - 4 = 2^x
   \]
   This equation has no integer solutions for \(x\) and \(k\).

5. **Conclusion:**
   The only solutions are when \(x = y\) and \(z = x + 1\), giving:
   \[
   y = \frac{x + z + 1}{2}, \quad u = 2^x + 2^z, \quad x \geq y \geq z
   \]

The final answer is \( \boxed{ y = \frac{x + z + 1}{2}, \quad u = 2^x + 2^z, \quad x \geq y \geq z } \)